Production of metal nanoparticles

ABSTRACT

A process for the production of metal nanoparticles. The process comprises a rapid mixing of a solution of at least about 0.1 mole of a metal compound that is capable of being reduced to a metal by a polyol with a heated solution of a polyol and a substance that is capable of being adsorbed on the nanoparticles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/331,230, filed Jan. 13, 2006, which claims the benefit of U.S.Provisional Application Ser. Nos. 60/643,577; 60/643,629; and60/643,578, all filed on Jan. 14, 2005, the entireties of which are allincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Agreement No.MDS972-93-2-0014 or DAAD1919-02-3-0001 awarded by the Army ResearchLaboratory (“ARL”). The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the production of metalnanoparticles. In particular, it relates to a process which affordsbetter control of the size, size distribution and/or shape of theparticles than the so-called polyol process.

2. Discussion of Background Information

The production of metal particles by the polyol process is known from,e.g., U.S. Pat. No. 4,539,041 to Figlarz et al., the entire disclosurewhereof is expressly incorporated by reference herein. In the polyolprocess, a metal compound is reduced at an elevated temperature by apolyol to afford the corresponding metal in the form of particles(usually in the micron and nanometer size range). A number of metalcompounds and in particular, a number of transition metal compounds canbe converted to metal particles by this process. In a typical procedure,a solid metal compound is suspended in a polyol and the suspension isheated until the reduction of the metal compound is substantiallycomplete. Thereafter, the formed particles are isolated by separatingthem from the liquid phase, e.g., by centrifugation.

A modification of this method is described in, e.g., P.-Y. Silvert etal., “Preparation of colloidal silver dispersions by the polyol process”Part 1—Synthesis and characterization, J. Mater. Chem., 1996, 6(4),573-577; and Part 2—Mechanism of particle formation, J. Mater. Chem.,1997, 7(2), 293-299. According to the Silvert et al. articles, theentire disclosures whereof are expressly incorporated by referenceherein, the polyol process is carried out in the presence of a polymer,i.e., polyvinylpyrrolidone (PVP). In particular, the PVP is dissolved inthe polyol and helps to control the size and the dispersity of the metalparticles. In a typical experiment, about 10 g of PVP was dissolved atroom temperature in 75 ml of ethylene glycol and 2.4 mmole (400 mg) ofsilver nitrate was added to this solution. The resultant suspension wasstirred at room temperature until the silver nitrate had dissolvedcompletely, whereafter the system was heated to 120° C. and the reactionwas conducted at this temperature for several hours. After cooling anddilution with water, the reaction mixture afforded silver particleshaving a mean particle size of 21 nm with a standard deviation of 16%.

While the reported results are desirable, the present inventors havefound that when the modified polyol process is scaled up and conductedwith a significantly larger amount of metal compound such as, e.g., 0.1mole of metal compound or more, in order to produce metal nanoparticlesin commercially significant amounts, the size and shape of the particlesbecomes non-uniform and the formation of large chunks, needle-likeparticles and the like is observed in addition to the formation ofsphere-like particles. Accordingly, it would be advantageous to haveavailable a process of the type described by Silvert et al. whichaffords satisfactory results in terms of particle size, shape and/orsize distribution even when it is conducted on a significantly largerscale than that reported by Silvert et al. Corresponding nanoparticleswould be useful in variety of applications. For example, there exists aneed for compositions for the fabrication of electrically conductivefeatures for use in electronics, displays, and other applications.Accordingly, nanoparticles produced by a process that affordscommercially significant amounts of substantially non-agglomerated,redispersible metal nanoparticles with a substantially uniform shape andsize could, for example, be used for the manufacture of printing inksand in particular, into inks for the printing of electrically conductivefeatures.

SUMMARY OF THE INVENTION

The present invention provides an improved polyol process for theproduction of metal nanoparticles. This process comprises the rapidmixing of a solution of at least about 0.1 mole of a metal compound thatis capable of being reduced to a metal by a polyol with a heatedsolution of a polyol and a substance that is capable of being adsorbedon the nanoparticles.

The present invention also provides a process for the production ofmetal nanoparticles which comprises a rapid mixing of a solution of atleast about 0.25 mole of a compound of at least one metal selected fromgold, silver, rhodium, palladium, platinum, copper, nickel and cobaltwith a heated solution that comprises a polyol and a polymer that iscapable of substantially preventing an agglomeration of thenanoparticles.

The present invention further provides a process for the production ofsilver nanoparticles which comprises a rapid mixing of a solution of atleast about 0.5 mole of a silver compound with a heated solution whichcomprises a polyol and a vinyl pyrrolidone polymer.

The present invention further provides a process for the production ofsilver nanoparticles which comprises a one-shot addition of a solutionof at least about 0.75 mole of a silver compound in ethylene glycoland/or propylene glycol to a heated solution of polyvinylpyrrolidone inethylene glycol and/or propylene glycol. The solution of the silvercompound is at a temperature of not higher than about 30° C., thepolyvinylpyrrolidone solution is at a temperature of at least about 110°C. The total volume of the solution of the silver compound and thepolyvinylpyrrolidone solution is from about 3 to about 4 liters per onemole of the silver compound and the volume ratio of thepolyvinylpyrrolidone solution and the solution of the silver compound isfrom about 4:1 to about 6:1. The molar ratio of vinyl pyrrolidone unitsin the polyvinylpyrrolidone and the silver compound is from about 5:1 toabout 15:1.

The present invention also provides a process for the production ofmetal nanoparticles which affords at least about 50 g of substantiallynon-agglomerated metal nanoparticles in a single run.

The present invention further provides a plurality of substantiallynon-agglomerated metal nanoparticles which are obtainable by theprocesses of the present invention, as well as a dispersion of thesenanoparticles which has a metal content of at least about 50 g/L andcomprises the nanoparticles in a substantially non-agglomerated state.

The present invention also provides process for producing an ink forink-jet printing, which process comprises combining these nanoparticleswith a liquid vehicle.

The present invention also provides a composition which is suitable forthe fabrication of an electrically conductive feature by using adirect-write tool. This composition comprises (a) the above metalnanoparticles and (b) a vehicle that is capable of forming a dispersionwith the metal nanoparticles.

The present invention also provides for the use of a composition for thefabrication of an electrically conductive feature, wherein thecomposition comprises (a) metal nanoparticles and (b) a vehicle that iscapable of forming a dispersion with the metal nanoparticles.

The present invention also provides a composition for the fabrication ofa conductive feature by ink-jet printing. This composition comprises (a)at least about 5 weight percent of silver nanoparticles which areobtainable by one of the processes of the present invention and (b) avehicle which comprises one or more organic solvents. The compositionhas a surface tension at 20° C. of from about 20 dynes/cm to about 40dynes/cm and a viscosity at 20° C. of from about 5 cP to about 15 cP.

The present invention also provides a composition for ink-jet printing.The composition comprises metal nanoparticles metal particles which areobtainable by one of the processes of the present invention, and iscapable of being deposited in not more than two passes of an ink-jetprinting head on a substrate as a line that can be rendered electricallyconductive.

The present invention further comprises a composition for providing asubstrate with a metal structure. The composition comprises (a) at leastabout 10 weight percent of silver nanoparticles which are obtainable byone of the processes of the present invention and havepolyvinylpyrrolidone thereon and (b) a vehicle which comprises anorganic solvent that is capable of dissolving polyvinylpyrrolidone. Thecomposition has a surface tension at 20° C. of not more than about 50dynes/cm and a viscosity at 20° C. of not higher than about 30 cP.

The present invention also provides a method for the fabrication of aconductive feature on a substrate, which method comprises forming thefeature by applying a composition of the present invention to thesubstrate and subjecting the feature to heat, pressure and/or radiationto render it conductive. The present invention also provides aconductive feature made by this process.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the embodiments of the present invention onlyand are presented in the cause of providing what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the present invention. In this regard, no attemptis made to set forth the present invention in more detail than isnecessary for the fundamental understanding of the present invention,the description making apparent to those skilled in the art how theseveral forms of the present invention may be embodied in practice.

According to the process of the present invention, a solution of atleast about 0.1 mole of a metal compound that is capable of beingreduced to a metal by a polyol (or other reductant) is rapidly mixedwith a heated solution of a polyol and a substance that is capable ofbeing adsorbed on the nanoparticles. It has been found that the controlof the size and/or the size distribution and/or the shape of thenanoparticles formed from the metal compound can be significantlyimproved when substantially the entire metal compound is employed indissolved form and contacted with the heated polyol within a relativelyshort period such as, e.g., within seconds. Compared to the standardpolyol process with its gradual dissolution/reaction of a solid metalcompound in a polyol, the advantages associated with the way ofcontacting the metal compound with the polyol according to the presentinvention are particularly pronounced when the amount of metal compoundis relatively large, i.e., where the required volume of the liquid phaseand/or the relatively high amount of metal compound per volume of theliquid phase makes it difficult, if not impossible, to avoid localconcentration gradients in the course of the dissolution of the metalcompound and/or to avoid inhomogeneous reaction conditions. Accordingly,the advantages associated with the process of the present invention interms of, e.g., particle size, particle size distribution, and/orparticle shape in comparison with the standard polyol process willusually become particularly pronounced if the amount of employed metalcompound is about 0.1 mole or higher, e.g., at least about 0.25 mole, atleast about 0.5 mole, at least about 0.75 mole, or at least about 1 moleand/or if the (initial) concentration of metal compound in the reactionmixture is at least about 0.1 mole, e.g., at least about 0.2 mole, atleast about 0.3 mole, or at least about 0.4 mole per liter of reactionmixture.

Metal Compound

The metal compounds that may be used in the process of the presentinvention include all metal compounds that a polyol can reduce to thecorresponding metal (oxidation state=0). Non-limiting examples of suchmetals include main group metals such as, e.g., lead, tin, antimony andindium, and transition metals such as, e.g., gold, silver, copper,nickel, cobalt, palladium, platinum, iridium, osmium, rhodium,ruthenium, rhenium, vanadium, chromium, manganese, niobium, molybdenum,tungsten, tantalum, iron and cadmium. Examples of preferred metalsinclude gold, silver, copper and nickel, in particular, silver, copperand nickel. Silver is a particularly preferred metal for the purposes ofthe present invention.

Since the metal compound is to be employed in dissolved form it shouldbe soluble to at least some extent in at least one solvent, preferably apolyol and/or a solvent that is substantially miscible with the heatedsolution. Also, the metal compound will usually be soluble to at leastsome extent in the polyol(s) of the heated solution so that there is nosubstantial precipitation or other separation of the metal compound fromthe liquid phase when the solution of the metal compound is contactedwith the heated solution.

Non-limiting examples of suitable metal compounds include metal oxides,metal hydroxides (including hydrated oxides), metal salts of inorganicand organic acids such as, e.g., nitrates, nitrites, sulfates, halides(e.g., fluorides, chlorides, bromides and iodides), carbonates,phosphates, azides, borates (including fluoroborates, pyrazolylborates,etc.), sulfonates, carboxylates (such as, e.g., formates, acetates,propionates, oxalates and citrates), substituted carboxylates (includinghalogenocarboxylates such as, e.g., trifluoroacetates,hydroxycarboxylates, aminocarboxylates, etc.) and salts and acidswherein the metal is part of an anion (such as, e.g.,hexachloroplatinates, tetrachloroaurate, tungstates and thecorresponding acids).

Further non-limiting examples of suitable metal compounds for theprocess of the present invention include alkoxides, complex compounds(e.g., complex salts) of metals such as, e.g., beta-diketonates (e.g.,acetylacetonates), complexes with amines, N-heterocyclic compounds(e.g., pyrrole, aziridine, indole, piperidine, morpholine, pyridine,imidazole, piperazine, triazoles, and substituted derivatives thereof),aminoalcohols (e.g., ethanolamine, etc.), amino acids (e.g., glycine,etc.), amides (e.g., formamides, acetamides, etc.), and nitriles (e.g.,acetonitrile, etc.). Non-limiting examples of preferred metal compoundsinclude nitrates, formates, acetates, trifluoroacetates, propionates,oxalates and citrates, particularly nitrates and acetates. Especiallyfor applications of the metal nanoparticles wherein a high electricalconductivity is a desired property, the metal compound is preferablysuch that the reduction by-product is volatile and/or can be decomposedinto a volatile by-product at a relatively low temperature. By way ofnon-limiting example, the reduction of a metal nitrate will usuallyresult in the formation of nitrogen oxide gases as the only by-products.

Non-limiting examples of specific metal compounds for use in the processof the present invention include silver nitrate, silver nitrite, silveroxide, silver fluoride, silver hydrogen fluoride, silver carbonate,silver oxalate, silver azide, silver tetrafluoroborate, silver acetate,silver propionate, silver butanoate, silver ethylbutanoate, silverpivalate, silver cyclohexanebutanoate, silver ethylhexanoate, silverneodecanoate, silver decanoate, silver trifluoroacetate, silverpentafluoropropionate, silver heptafluorobutyrate, silvertrichloroacetate, silver6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate, silverlactate, silver citrate, silver glycolate, silver glyconate, silverbenzoate, silver salicylate, silver phenylacetate, silvernitrophenylacetate, silver dinitrophenylacetate, silverdifluorophenylacetate, silver 2-fluoro-5-nitrobenzoate, silveracetylacetonate, silver hexafluoroacetylacetonate, silvertrifluoroacetylacetonate, silver tosylate, silver triflate, silvertrispyrazolylborate, silver tris(dimethylpyrazolyl)borate, silver aminecomplexes, trialkylphosphine and triarylphosphine derivatives of silvercarboxylates, silver beta-diketonates, silver beta-diketonate olefincomplexes and silver cyclopentadienides; nickel oxide, nickel hydroxide,nickel chloride, nickel nitrate, nickel sulfate, nickel amine complexes,nickel tetrafluoroborate, nickel oxalate, nickel isopropoxide, nickelmethoxyethoxide, nickel acetylacetonate, nickel formate, nickel acetate,nickel octanoate, nickel ethylhexanoate, and nickel trifluoroacetate;platinum formate, platinum acetate, platinum propionate, platinumcarbonate, platinum nitrate, platinum perchlorate, platinum benzoate,platinum neodecanoate, platinum oxalate, ammonium hexafluoroplatinate,ammonium tetrachloroplatinate, sodium hexafluoroplatinate, potassiumhexafluoroplatinate, sodium tetrachloroplatinate, potassiumhexabromoplatinate, hexachloroplatinic acid, hexabromoplatinic acid,dihydrogen hexahydroxoplatinate, diamine platinum chloride, tetraamineplatinum chloride, tetraamine platinum hydroxide, tetraamine platinumtetrachloroplatinate, platinum(II) 2,4-pentanedionate, diplatinumtrisdibenzylideneacetonate, platinum sulfate and platinumdivinyltetramethyldisiloxane; gold(III) acetate, gold(III) chloride,tetrachloroauric acid, gold azide, gold isocyanide, gold acetoacetate,imidazole gold ethylhexanoate and gold hydroxide acetate isobutyrate;palladium acetate, palladium propionate, palladium ethylhexanoate,palladium neodecanoate, palladium trifluoracetate, palladium oxalate,palladium nitrate, palladium chloride, tetraamine palladium hydroxide,tetraamine palladium nitrate and tetraamine palladiumtetrachloropalladate; copper oxide, copper hydroxide, copper nitrate,copper sulfate, copper chloride, copper formate, copper acetate, copperneodecanoate, copper ethylhexanoate, copper methacrylate, coppertrifluoroacetate, copper acetoacetate and copperhexafluoroacetylacetonate; cobalt oxide, cobalt hydroxide, cobaltchloride and cobalt sulfate; ruthenium(III) chloride, ruthenium(III)acetylacetonate, ruthenium(III) acetate, ruthenium carbonyl complexes,ruthenium perchlorate, and ruthenium amine complexes; rhodium(III)chloride, rhenium(II) chloride, tin(II) oxide, iron(II) acetate, sodiumtungstate and tungstic acid. The above compounds may be employed as suchor optionally in the form of solvates and the like such as, e.g., ashydrates.

Examples of preferred metal compounds for use in the present inventioninclude silver nitrate, silver acetate, silver trifluoroacetate, silveroxide, copper oxide, copper hydroxide, copper sulfate, nickel oxide,nickel hydroxide, nickel chloride, nickel sulfate, nickel acetate,cobalt oxide, cobalt hydroxide, cobalt chloride and cobalt sulfate.

The use of mixtures of different compounds, e.g., different salts, ofthe same metal and/or the use of mixtures of compounds of differentmetals and/or of mixed metal compounds (e.g., mixed salts and/or mixedoxides) are also contemplated by the present invention. Accordingly, theterm “metal compound” as used herein includes both a single metalcompound and any mixture of two or more metal compounds. Depending,inter alia, on the metal compounds and reaction conditions employed, theuse of more than one metal in the process of the present invention willresult in a mixture of nanoparticles of different metals and/or innanoparticles which comprise different metals in the same nanoparticle,for example, in the form of an alloy or a mixture of these metals.Non-limiting examples of alloys include Ag/Ni, Ag/Cu, Pt/Cu, Ru/Pt,Ir/Pt and Ag/Co.

Polyol

The polyol for use in the present invention may be a single polyol or amixture of two or more polyols (e.g., three, four or five polyols). Inthe following description, whenever the term “polyol” is used, this termis meant to include both a single polyol and a mixture of two or morepolyols. The polyol may have any number of hydroxyl groups (but at leasttwo) and carbon atoms. Also, the polyol may comprise heteroatoms (suchas, e.g., O and N), not only in the form of hydroxyl groups, but also inthe form of, e.g., ether, ester, amine and amide groups and the like(for example, it may be a polyester polyol, a polyether polyol, etc.).Preferably, the polyol comprises from about 2 to about 6 hydroxy groups(e.g., 2, 3 or 4 hydroxy groups). Also, the preferred polyol comprisesfrom 2 to about 12 carbon atoms, e.g., up to about 3, 4, 5 or 6 carbonatoms. A particularly preferred group of polyols for use in the presentinvention are the (alkylene) glycols, i.e., compounds which comprise twohydroxyl groups bound to adjacent (aliphatic or cycloaliphatic) carbonatoms. Usually these glycols will comprise up to about 6 carbon atoms,e.g., 2, 3 or 4 carbon atoms. Ethylene glycol, propylene glycol and thebutylene glycols are non-limiting examples of preferred glycols for usein the present invention.

The polyglycols constitute another group of preferred polyols for use inthe present invention. Specific and preferred examples thereof arecompounds which comprise up to about 6 alkylene glycol units, e.g., upto 4 alkylene glycol units, such as, e.g., diethylene glycol,triethylene glycol, tetraethylene glycol, dipropylene glycol andtripropylene glycol.

Non-limiting examples of other specific compounds which mayadvantageously be used as the or a polyol in the process of the presentinvention include 1,3-propanediol, 1,3-butanediol, 1,4-butanediol,glycerol, trimethylolpropane, pentaerythritol, triethanolamine andtrihydroxymethylaminomethane.

Of course, it also is possible to use other polyols than those mentionedabove, either alone or in combination. For example, sugars and sugaralcohols can form at least a part of the polyol reactant of the processof the present invention. While polyols that are solid or semi-solid atroom temperature may be employed, it is preferred that the employedpolyol or at least the employed mixture of polyols is liquid at roomtemperature, although this is not mandatory. Further, it is alsopossible to use one or more other reducing agents in conjunction withthe polyol(s), for example, in order to reduce the required reactiontime and/or the reaction temperature. For instance, the substance thatis capable of being adsorbed on the nanoparticles may exhibit a reducingeffect with respect to the metal compound. A non-limiting example ofsuch a substance is polyvinylpyrrolidone. Non-limiting examples of otherreducing agents which may be employed in accordance with the presentinvention include hydrazine and derivatives thereof, hydroxylamine andderivatives thereof, aldehydes such as, e.g., formaldehyde,hypophosphites, sulfites, tetrahydroborates (such as, e.g., thetetrahydroborates of Li, Na, K), LiAlH₄, polyhydroxybenzenes such as,e.g., hydroquinone, alkyl-substituted hydroquinones, catechols andpyrogallol; phenylenediamines and derivatives thereof; aminophenols andderivatives thereof; ascorbic acid and ascorbic acid ketals and otherderivatives of ascorbic acid; 3-pyrazolidone and derivatives thereof;hydroxytetronic acid, hydroxytetronamide and derivatives thereof;bisnaphthols and derivatives thereof; sulfonamidophenols and derivativesthereof; and Li, Na and K.

Adsorptive Substance

One of the functions of the substance that is capable of being adsorbedon the nanoparticles (hereafter frequently referred to as “theadsorptive substance”) will usually and preferably be to help prevent asubstantial agglomeration of the nanoparticles. Due to their small sizeand the high surface energy associated therewith, the metalnanoparticles exhibit a strong tendency to agglomerate and form largersecondary particles (for example, soft agglomerates). The adsorptivesubstance will shield (e.g., sterically and/or through charge effects)the nanoparticles from each other to at least some extent and therebysubstantially reduce or prevent a direct contact between the individualnanoparticles. The term “adsorbed” as used herein and in the appendedclaims includes any kind of interaction between the adsorptive substanceand a nanoparticle surface (e.g., the metal atoms on the surface of ananoparticle) that manifests itself in an at least weak bond between theadsorptive substance and the surface of a nanoparticle. Preferably, thebond is strong enough for the nanoparticle-adsorptive substancecombination to withstand a washing operation with a solvent for theadsorptive substance. In other words, merely washing the nanoparticleswith the solvent at room temperature will preferably not remove morethan minor amounts (e.g., less than about 10%, less than about 5%, orless than about 1%) of that part of the adsorptive substance that is indirect contact with (and (weakly) bonded to) the nanoparticle surface.Of course, adsorptive substance that is not in direct contact with ananoparticle surface and is merely associated with the bulk of thenanoparticles as a contaminant, i.e., without any significantinteraction with the nanoparticles, is preferably removable from thenanoparticles by washing the latter with a solvent for the adsorptivesubstance. Further, it is also preferred for the bond between theadsorptive substance and nanoparticle to be not too strong and, inparticular, to not be a covalent bond.

While the adsorptive substance will usually be a single substance or atleast comprise substances of the same type, the present invention alsocontemplates the use of two or more different types of adsorptivesubstances. For example, a mixture of two or more different lowmolecular weight compounds or a mixture of two or more differentpolymers may be used, as well as a mixture of one or more low molecularweight compounds and one or more polymers. The terms “substance that iscapable of being adsorbed on the nanoparticles” and “adsorptivesubstance” as used herein include all of these possibilities. One ofskill in the art will understand that volatile components of the mixturesuch as, e.g., the polyol and/or solvent may also have a tendency ofbeing adsorbed on the nanoparticle surface. These substances do notqualify as “adsorptive substances” within the meaning of this term asused herein.

The adsorptive substance should preferably be compatible with the polyolin the heated solution, i.e., it preferably does not react with thepolyol to any significant extent, even at the elevated temperatures thatwill often be employed in the process of the present invention. If theheated solution does not comprise any other solvent for the adsorptivesubstance, the substance should also dissolve in the polyol to at leastsome extent. The adsorptive substance will usually have a solubility atroom temperature of at least about 1 g per liter of solvent (includingsolvent mixtures), e.g., at least about 5 g, at least about 10 g, or atleast about 20 g per liter of solvent. Preferably, the adsorptivesubstance has a solubility of at least about 100 g, e.g., at least about200 g, or at least about 300 g per liter of solvent.

A preferred and non-limiting example of an adsorptive substance for usein the process of the present invention includes a substance that iscapable of electronically interacting with a metal atom of ananoparticle. Usually, a substance that is capable of this type ofinteraction will comprise one or more atoms (e.g., at least two atoms)with one or more free electron pairs such as, e.g., oxygen, nitrogen andsulfur. By way of non-limiting example, the adsorptive substance may becapable of a dative interaction with a metal atom on the surface of ananoparticle and/or of chelating the metal atom. Particularly preferredadsorptive substances comprise one or two O and/or N atoms. The atomswith a free electron pair will usually be present in the substance inthe form of a functional group such as, e.g., a hydroxy group, acarbonyl group, an ether group and an amino group, or as a constituentof a functional group that comprises one or more of these groups as astructural element thereof. Non-limiting examples of suitable functionalgroups include —COO—, —O—CO—O—, —CO—O—CO—, —C—O—C—, —CONR—, —NR—CO—O—,—NR¹—CO—NR²—, —CO—NR—CO—, —SO₂—NR— and —SO₂—O—, wherein R, R¹ and R²each independently represent hydrogen or an organic radical (e.g., analiphatic or aromatic, unsubstituted or substituted radical comprisingfrom about 1 to about 20 carbon atoms). Such functional groups maycomprise the above (and other) structural elements as part of a cyclicstructure (e.g., in the form of a cyclic ester, amide, anhydride, imide,carbonate, urethane, urea, and the like).

In one aspect of the process of the present invention, the adsorptivesubstance is or comprises a substance that is capable of reducing themetal compound, i.e., in addition to the reduction by the polyol used. Aspecific, non-limiting example of such a substance ispolyvinylpyrrolidone (PVP).

The adsorptive substance may comprise a low molecular weight compound,preferably a low molecular weight organic compound, e.g., a compoundhaving a molecular weight of not higher than about 500, more preferablynot higher than about 300, and/or may comprise an oligomeric orpolymeric compound having a (weight average) molecular weight (inDaltons) of at least about 1,000, for example, at least about 3,000, atleast about 5,000, or at least about 8,000, but preferably not higherthan about 500,000, e.g., not higher than about 200,000, or not higherthan about 100,000. Too high a molecular weight may give rise to anundesirably high viscosity of the solution at a desirable concentrationof the adsorptive substance and/or cause flocculation. Also, the mostdesirable molecular weight may be dependent on the metal. By way ofnon-limiting example, in the case of polyvinylpyrrolidone, aparticularly preferred weight average molecular weight is in the rangeof from about 3,000 to about 60,000, in particular if the metalcomprises silver.

In general, it is preferred for the adsorptive substance to have a totalof at least about 10 atoms per molecule which are selected from C, N andO, e.g., at least about 20 such atoms or at least about 50 such atoms.More preferably, the adsorptive substance has a total of at least about100 C, N and O atoms per molecule, e.g., at least about 200, at leastabout 300, or at least about 400 C, N and O atoms per molecule. In thecase of polymers these numbers refer to the average per polymermolecule.

Non-limiting examples of the low molecular weight adsorptive substancefor use in the present invention include fatty acids, in particular,fatty acids having at least about 8 carbon atoms. Non-limiting examplesof oligomers/polymers for use as the adsorptive substance in the processof the present invention include homo- and copolymers (includingpolymers such as, e.g., random copolymers, block copolymers and graftcopolymers) which comprise units of at least one monomer which comprisesone or more O atoms and/or one or more N atoms. A non-limiting class ofpreferred polymers for use in the present invention is constituted bypolymers which comprise at least one monomer unit which includes atleast two atoms which are selected from O and N atoms. Correspondingmonomer units may, for example, comprise at least one hydroxyl group,carbonyl group, ether linkage and/or amino group and/or one or morestructural elements of formula —COO—, —O—CO—O—, —CO—O—CO—, —C—O—C—,—CONR—, —NR—CO—O—, NR²—, —CO—NR—CO—, —SO₂—NR— and —SO₂—O—, wherein R, R¹and R² each independently represent hydrogen or an organic radical(e.g., an aliphatic or aromatic, unsubstituted or substituted radicalcomprising from about 1 to about 20 carbon atoms).

Non-limiting examples of corresponding polymers include polymers whichcomprise one or more units derived from the following groups ofmonomers:

(a) monoethylenically unsaturated carboxylic acids of from about 3 toabout 8 carbon atoms and salts thereof. This group of monomers includes,for example, acrylic acid, methacrylic acid, dimethylacrylic acid,ethacrylic acid, maleic acid, citraconic acid, methylenemalonic acid,allylacetic acid, vinylacetic acid, crotonic acid, fumaric acid,mesaconic acid and itaconic acid. The monomers of group (a) can be usedeither in the form of the free carboxylic acids or in partially orcompletely neutralized form. For the neutralization alkali metal bases,alkaline earth metal bases, ammonia or amines, e.g., sodium hydroxide,potassium hydroxide, sodium carbonate, potassium carbonate, sodiumbicarbonate, magnesium oxide, calcium hydroxide, calcium oxide, ammonia,triethylamine, methanolamine, diethanolamine, triethanolamine,morpholine, diethylenetriamine or tetraethylenepentamine may, forexample, be used;(b) the esters, amides, anhydrides and nitriles of the carboxylic acidsstated under (a) such as, e.g., methyl acrylate, ethyl acrylate, methylmethacrylate, ethyl methacrylate, n-butyl acrylate, hydroxyethylacrylate, 2- or 3-hydroxypropyl acrylate, 2- or 4-hydroxybutyl acrylate,hydroxyethyl methacrylate, 2- or 3-hydroxypropyl methacrylate,hydroxyisobutyl acrylate, hydroxyisobutyl methacrylate, monomethylmaleate, dimethyl maleate, monoethyl maleate, diethyl maleate, maleicanhydride, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, acrylamide,methacrylamide, N,N-dimethylacrylamide, N-tert-butylacrylamide,acrylonitrile, methacrylonitrile, 2-dimethylaminoethyl acrylate,2-dimethylaminoethyl methacrylate, 2-diethylaminoethyl acrylate,2-diethylaminoethyl methacrylate and the salts of the last-mentionedmonomers with carboxylic acids or mineral acids and the quaternizedproducts;(c) acrylamidoglycolic acid, vinylsulfonic acid, allylsulfonic acid,methallylsulfonic acid, styrenesulfonic acid, 3-sulfopropyl acrylate,3-sulfopropyl methacrylate and acrylamidomethylpropanesulfonic acid andmonomers containing phosphonic acid groups, such as, e.g., vinylphosphate, allyl phosphate and acrylamidomethylpropanephosphonic acid;and esters, amides and anhydrides of these acids;(d) N-vinyllactams such as, e.g., N-vinylpyrrolidone,N-vinyl-2-piperidone and N-vinylcaprolactam;(e) vinyl acetal, vinyl butyral, vinyl alcohol and ethers and estersthereof (such as, e.g., vinyl acetate, vinyl propionate andmethylvinylether), allyl alcohol and ethers and esters thereof;N-vinylimidazole, N-vinyl-2-methylimidazoline, and the hydroxystyrenes.

Corresponding polymers may also contain additional monomer units, forexample, units derived from monomers without functional group,halogenated monomers, aromatic monomers etc. Non-limiting examples ofsuch monomers include olefins such as, e.g., ethylene, propylene, thebutenes, pentenes, hexenes, octenes, decenes and dodecenes, styrene,vinyl chloride, vinylidene chloride, tetrafluoroethylene, etc. Further,the polymers for use as adsorptive substance in the process of thepresent invention are not limited to addition polymers, but alsocomprise other types of polymers, for example, condensation polymerssuch as, e.g., polyesters, polyamides, polyurethanes and polyethers, aswell as polysaccharides such as, e.g., starch, cellulose and derivativesthereof, etc.

Other non-limiting examples of polymers which are suitable for use asadsorptive substance in the present invention are disclosed in e.g.,U.S. Patent Application Publication 2004/0182533 A1, the entiredisclosure whereof is expressly incorporated by reference herein.

Preferred polymers for use as adsorptive substance in the presentinvention include those which comprise units derived from one or moreN-vinylcarboxamides of formula (I)

CH₂═CH—NR³—CO—R⁴  (I)

wherein R³ and R⁴ independently represent hydrogen, optionallysubstituted alkyl (including cycloalkyl) and optionally substituted aryl(including alkaryl and aralkyl) or heteroaryl (e.g., C₆₋₂₀ aryl such asphenyl, benzyl, tolyl and phenethyl, and C₄₋₂₀ heteroaryl such aspyrrolyl, furyl, thienyl and pyridinyl).

R³ and R⁴ may, e.g., independently represent hydrogen or C₁₋₁₂ alkyl,particularly C₁₋₆ alkyl such as methyl and ethyl. R³ and R⁴ together mayalso form a straight or branched chain containing from about 2 to about8, preferably from about 3 to about 6, particularly preferably fromabout 3 to about 5 carbon atoms, which chain links the N atom and the Catom to which R³ and R⁴ are bound to form a ring which preferably hasabout 4 to about 8 ring members. Optionally, one or more carbon atomsmay be replaced by heteroatoms such as, e.g., oxygen, nitrogen orsulfur. Also optionally, the ring may contain a carbon-carbon doublebond.

Non-limiting specific examples of R³ and R⁴ are methyl, ethyl,isopropyl, n-propyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-hexyl,n-heptyl, 2-ethylhexyl, n-octyl, n-decyl, n-undecyl, n-dodecyl,n-tetradecyl, n-hexadecyl, n-octadecyl and n-eicosyl. Non-limitingspecific examples of R³ and R⁴ which together form a chain are1,2-ethylene, 1,2-propylene, 1,3-propylene, 2-methyl-1,3-propylene,2-ethyl-1,3-propylene, 1,4-butylene, 1,5-pentylene,2-methyl-1,5-pentylene, 1,6-hexylene and 3-oxa-1,5-pentylene.

Non-limiting specific examples of N-vinylcarboxamides of formula (I) areN-vinylformamide, N-vinylacetamide, N-vinylpropionamide,N-vinylbutyramide, N-vinylisobutyramide, N-vinyl-2-ethylhexanamide,N-vinyldecanamide, N-vinyldodecanamide, N-vinylstearamide,N-methyl-N-vinylformamide, N-methyl-N-vinylacetamide,N-methyl-N-vinylpropionamide, N-methyl-N-vinylbutyramide,N-methyl-N-vinylisobutyramide, N-methyl-N-vinyl-2-ethylhexanamide,N-methyl-N-vinyldecanamide, N-methyl-N-vinyldodecanamide,N-methyl-N-vinylstearamide, N-ethyl-N-vinylformamide,N-ethyl-N-vinylacetamide, N-ethyl-N-vinylpropionamide,N-ethyl-N-vinylbutyramide, N-ethyl-N-vinylisobutyramide,N-ethyl-N-vinyl-2-ethylhexanamide, N-ethyl-N-vinyldecanamide,N-ethyl-N-vinyldodecanamide, N-ethyl-N-vinylstearamide,N-isopropyl-N-vinylformamide, N-isopropyl-N-vinylacetamide,N-isopropyl-N-vinylpropionamide, N-isopropyl-N-vinylbutyramide,N-isopropyl-N-vinylisobutyramide, N-isopropyl-N-vinyl-2-ethylhexanamide,N-isopropyl-N-vinyldecanamide, N-isopropyl-N-vinyldodecanamide,N-isopropyl-N-vinylstearamide, N-n-butyl-N-vinylformamide,N-n-butyl-N-vinylacetamide, N-n-butyl-N-vinylpropionamide,N-n-butyl-N-vinylbutyramide, N-n-butyl-N-vinylisobutyramide,N-n-butyl-N-vinyl-2-ethylhexanamide, N-n-butyl-N-vinyldecanamide,N-n-butyl-N-vinyldodecanamide, N-n-butyl-N-vinylstearamide,N-vinylpyrrolidone, N-vinyl-2-piperidone and N-vinylcaprolactam.

Particularly preferred polymers for use in the present invention includepolymers which comprise monomer units of one or more unsubstituted orsubstituted N-vinyllactams, preferably those having from about 4 toabout 8 ring members such as, e.g., N-vinylcaprolactam,N-vinyl-2-piperidone and N-vinylpyrrolidone. These polymers includehomo- and copolymers. In the case of copolymers (including, for example,random, block and graft copolymers), the N-vinyllactam (e.g.,N-vinylpyrrolidone) units are preferably present in an amount of atleast about 10 mole-%, e.g., at least about 30 mole-%, at least about 50mole-%, at least about 70 mole-%, at least about 80 mole-%, or at leastabout 90 mole-%. By way of non-limiting example, the comonomers maycomprise one or more of those mentioned in the preceding paragraphs,including monomers without functional group (e.g., ethylene, propylene,styrene, etc.), halogenated monomers, etc.

If the vinyllactam (e.g., vinylpyrrolidone) monomers (or at least a partthereof) carry one or more substituents on the heterocyclic ring,non-limiting examples of such substituents include alkyl groups (forexample, alkyl groups having from 1 to about 12 carbon atoms, e.g., from1 to about 6 carbon atoms such as, e.g., methyl, ethyl, propyl andbutyl), alkoxy groups (for example, alkoxy groups having from 1 to about12 carbon atoms, e.g., from 1 to about 6 carbon atoms such as, e.g.,methoxy, ethoxy, propoxy and butoxy), halogen atoms (e.g., F, Cl andBr), hydroxy, carboxy and amino groups (e.g., dialkylamino groups suchas dimethylamino and diethylamino) and any combinations of thesesubstituents.

Non-limiting specific examples of vinyllactam polymers for use in thepresent invention include homo- and copolymers of vinylpyrrolidone whichare commercially available from, e.g., International Specialty Products(www.ispcorp.com). In particular, these polymers include

(a) vinylpyrrolidone homopolymers such as, e.g., grades K-15 and K-30with K-value ranges of from 13-19 and 26-35, respectively, correspondingto average molecular weights (determined by GPC/MALLS) of about 10,000and about 67,000;(b) alkylated polyvinylpyrrolidones such as, e.g., those commerciallyavailable under the trade mark GANEX® which arevinylpyrrolidone-alpha-olefin copolymers that contain most of thealpha-olefin (e.g., about 80% and more) grafted onto the pyrrolidonering, mainly in the 3-position thereof; the alpha-olefins may comprisethose having from about 4 to about 30 carbon atoms; the alpha-olefincontent of these copolymers may, for example, be from about 10% to about80% by weight;(c) vinylpyrrolidone-vinylacetate copolymers such as, e.g., randomcopolymers produced by a free-radical polymerization of the monomers ina molar ratio of from about 70/30 to about 30/70 and having weightaverage molecular weights of from about 14,000 to about 58,000;(d) vinylpyrrolidone-dimethylaminoethylmethacrylate copolymers;(e) vinylpyrrolidone-methacrylamidopropyl trimethylammonium chloridecopolymers such as, e.g., those commercially available under the trademark GAFQUAT®;(f) vinylpyrrolidone-vinylcaprolactam-dimethylaminoethylmethacrylateterpolymers such as, e.g., those commercially available under the trademark GAFFIX®;(g) vinylpyrrolidone-styrene copolymers such as, e.g., thosecommercially available under the trade mark POLECTRON®; a specificexample thereof is a graft emulsion copolymer of about 70%vinylpyrrolidone and about 30% styrene polymerized in the presence of ananionic surfactant;(h) vinylpyrrolidone-acrylic acid copolymers such as, e.g., thosecommercially available under the trade mark ACRYLIDONE® which areproduced in the molecular weight range of from about 80,000 to about250,000.

Solvent for Metal Compound

The solvent for the metal compound may be a single solvent or a mixtureof two or more solvents/diluents (collectively referred to herein as“solvent” or “solvent for the metal compound”). The solvent ispreferably capable of dissolving a significant amount of the metalcompound at room temperature and/or at the temperature that the solutionof the metal compound is intended to have when it is combined with theheated polyol solution. Usually the solvent will dissolve the metalcompound at room temperature in an amount of at least about 1 g/l, e.g.,at least about 5 g/l, or at least about 10 g/l. Preferably, the metalcompound dissolves in the solvent in an amount of at least about 50 g/l,e.g., at least about 100 g/l, at least about 200 g/l, or at least about300 g/l. In this regard, it is to be appreciated that one or morecomponents of the solvent may be poor solvents for the metal compound aslong as the entirety of the solvent, i.e., all components thereof, arecapable of dissolving the metal compound to the desired extent. Thesolvent for the metal compound should preferably also be miscible withthe polyol of the heated solution to at least some extent. Also, thesolvent for the metal compound is preferably of high purity. Thisapplies also to any other liquids/solvents that are used in the processof the present invention.

It is to be understood that the solution of the metal compound may stillcontain some undissolved metal compound, although this is not preferred.By way of non-limiting example, not more than about 20 weight percent,e.g., not more than about 10 weight percent, or not more than about 5weight percent, of the metal compound may be present in undissolvedform. Preferably, the amount of undissolved metal compound is not higherthan about 1 weight percent, even more preferred not higher than about0.1 weight percent. Most preferably, the solution is substantially freeof undissolved metal compound (e.g., not higher than about 0.01 weightpercent, or not higher than about 0.001 weight percent of undissolvedmetal compound). If undissolved metal compound is present, it ispreferred for it to be dissolved substantially immediately upon contactwith the heated solution, for example, within less than about one minute(e.g., less than about 30 seconds). Especially in cases where thetemperature difference between the solution of the metal compound andthe heated solution is large (e.g., larger than about 40° C.)., it maybe particularly advantageous to employ a highly concentrated and inparticular, a substantially saturated solution of the metal compound(preferably in a good solvent therefor) in order to keep the temperaturedrop relative to the temperature of the heated solution upon combiningthe solutions small.

In a preferred aspect of the method of the present invention, thesolvent for the metal compound is or at least comprises one or morepolyols, preferably the same polyol(s) that is/are present in the heatedsolution. It is noted however, that the use of one or more polyols fordissolving the metal compound is not mandatory. Other solvents may beused as well, such as, e.g., protic and aprotic polar solvents.Non-limiting examples of such solvents include aliphatic, cycloaliphaticand aromatic alcohols (the term “alcohol” as used herein is usedinterchangeably with the terms “monoalcohol” and “monohydric alcohol”)such as, e.g., ethanol, propanol, butanol, pentanol, cyclopentanol,hexanol, cyclohexanol, octanol, decanol, isodecanol, undecanol,dodecanol, benzyl alcohol, butyl carbitol and the terpineols, etheralcohols such as, e.g., the monoalkyl ethers of diols such as, e.g., theC₁₋₆ monoalkyl ethers of C₁₋₆ alkanediols and polyetherdiols derivedtherefrom (e.g., the monomethyl, monoethyl, monopropyl and monobutylethers of ethylene glycol, diethylene glycol, triethylene glycol,propylene glycol, dipropylene glycol, 1,3-propanediol, and1,4-butanediol such as, e.g, 2-methoxyethanol, 2-ethoxyethanol,2-propoxyethanol and 2-butoxyethanol), aminoalcohols such as, e.g.,ethanolamine, amides such as, e.g., dimethylformamide, dimethylacetamide2-pyrrolidone and N-methylpyrrolidone, esters such as, e.g., ethylacetate and ethyl formate, sulfoxides such as, e.g., dimethylsulfoxide,ethers such as, e.g., tetrahydrofuran and tetrahydropyran, and water.These and other suitable solvents may be used alone or as a mixture oftwo or more thereof and/or in combination with one or more polyols. Bythe same token, it is possible for the heated solution to comprise oneor more solvents in addition to the one or more polyols includedtherein. By way of non-limiting example, the heated solution mayadditionally comprise solvents such as those that may be present in thesolution of the metal compound. However, in the combined solutions(solution of metal compound and heated solution) the concentration ofpolyol(s) should be sufficiently high to bring about a reduction of atleast a substantial portion (and preferably, substantially all) of themetal compound within a commercially acceptable period of time (forexample, within not more than about 24 hours, preferably not more thanabout 12 hours or not more than about 6 hours) when the mixture isheated to a temperature that does not cause a substantial decompositionof any of the components that are present in the mixture.

Temperature

The temperature of the heated solution is preferably at least about 60°C., for example, at least about 70° C., at least about 80° C., at leastabout 85° C., at least about 90° C., at least about 100° C., at leastabout 110° C., or at least about 120° C. On the other hand, thetemperature of the heated solution will usually be not higher than about180° C., e.g., not higher than about 170° C., not higher than about 160°C., not higher than about 150° C., or not higher than about 140° C. Themost suitable temperature of the heated solution is at least in partdetermined by factors such as the boiling point of the solvent(s)included therein (i.e., the boiling point of at least the polyol), thethermal stability of the adsorptive substance, the reactivities of themetal compound and the polyol, and the temperature of the solution ofthe metal compound and the volume thereof relative to the heatedsolution.

The temperature of the second solution used in the process of thepresent invention, i.e., the solution of the metal compound, willusually be not higher than that of the heated solution and willfrequently be not higher than about 50° C., e.g., not higher than about40° C., or not higher than about 30° C. On the other hand, too low atemperature may increase the viscosity of the solution and/or reduce thesolubility of the metal compound to an undesirable degree. Usually, thetemperature of the solution will be about room temperature. Particularlyin cases where the metal compound is dissolved in a solvent thatcomprises one or more polyols which are capable of reducing the metalcompound, it is advantageous to keep the solvent at a temperature atwhich the rate of the reaction between the metal compound and the polyolis low in order to substantially prevent a reduction of the metalcompound by the polyol component while the metal compound or at least apart thereof is still present in the solid state. Even if all or almostall of the metal compound is present in dissolved form, a substantialreaction thereof with the polyol component in the absence of theadsorptive substance is undesirable, wherefore the temperature of apolyol containing solution of the metal compound prior to thecombination thereof with the heated solution should preferably be keptlow, in particular when highly reactive polyols and/or highly reactivemetal compounds are present. One of skill in the art will appreciatethat if the solution of the metal compound does not comprise a substance(solvent) that will reduce or otherwise react with the metal compound toany substantial extent even at an elevated temperature, the temperatureof the solution of the metal compound may even be higher than that ofthe heated solution and will preferably be close to (e.g., within about10° C.), for example, about the same as the temperature of the heatedsolution so that upon mixing these two solution there is substantiallyno temperature change in the system.

In a preferred aspect of the process of the present invention, thetemperature of the solution of the metal compound will be notsubstantially higher than about 40° C. and the heated solution will beat a temperature of at least about 80° C.

Mixing

The rate at which the solution of the metal compound and the heatedsolution are combined in the process of the present invention ispreferably as high as possible. By way of non-limiting example, the twosolutions will usually be completely combined within not more than about5 minutes, preferably within not more than about 2 minutes, e.g., withinnot more than about 1 minute, within not more than about 30 seconds,within not more than about 15 seconds, or within not more than about 5seconds. Most preferably, the solutions are combined virtuallyinstantaneously, such as by a one-shot addition of one of the solutionsto the other solution, e.g., by a one-shot addition of the solution ofthe metal compound to the heated solution.

It is also preferred according to the present invention to promote thecomplete mixing of the two solutions, for example, by agitation such as,e.g., by (preferably vigorous) stirring, shaking and/or sonication ofthe combined solutions.

Ratio and Total Volume of Solutions

The total volume of the solution of the metal compound and the heatedsolution are not particularly limited. However, with increasing amountsof the employed metal compound it will become increasingly desirable tokeep the total volume of the solutions small so as to keep the volume ofthe reaction mixture and, thus the required size of the reaction vesselas small as possible and also to keep the amount of liquids that need tobe discarded/recycled after the reaction is completed at a minimum.Accordingly, provided the solubility of the metal compound in theselected liquid components of the reaction mixture is high enough, thecombined volume of the two solutions per one mole of employed metalcompound will usually be not larger than about 10 liters, e.g., notlarger than about 8 liters, not larger than about 6 liters, not largerthan about 5 liters, or not larger than about 4 liters. On the otherhand, for reasons of, inter alia, solubility, the combined volume willusually be not smaller than about 1 liter, e.g., not smaller than about2 liters, or not smaller than about 3 liters per one mole of employedmetal compound. Of course, (considerably) smaller or larger combinedvolumes than those indicated herein may sometimes be more desirableand/or advantageous.

The volume ratio of the two solutions is not particularly limited,either. The most desirable volume ratio is influenced by several factorssuch as, e.g., the volume of solvent that is to dissolve the metalcompound, the temperature difference between the solutions, and thedesire to keep the total volume of the liquid phase as low as possible(e.g., for the reasons stated above). In many cases the volume ratio ofthe heated solution and the solution of the metal compound will be nothigher than about 10:1, e.g., not higher than about 8:1, or not higherthan about 6:1. On the other hand, the volume ratio will often be notlower than about 1:1, e.g., not lower than about 2:1, not lower thanabout 3:1, or not lower than about 4:1. Of course, there may besituations where lower or higher volume ratios than those indicatedherein may be more advantageous and/or desirable. Especially in caseswhere the temperature difference between the solution of the metalcompound and the heated solution is large (e.g., higher than about 40°C.), it will often be advantageous to use a relatively high volume ratioin order to keep the temperature drop relative to the temperature of theheated solution upon combining the solutions small. This may beaccomplished, for example, by employing a concentrated (e.g., saturated)solution of the metal compound in a good solvent therefor.

Ratio of Metal Compound and Adsorptive Substance

The most desirable ratio of the metal compound and the adsorptivesubstance is a function of a variety of factors. In this regard, it isto be appreciated that the adsorptive substance will generally havemultiple functions. These functions include, of course, a substantialprevention of an agglomeration of the nanoparticles and, as a resultthereof, facilitating an isolation of the nanoparticles from thereaction mixture, ensuring a substantial redispersibility of theisolated nanoparticles and a stabilization of dispersions comprisingthese nanoparticles. Another function of the adsorptive substanceusually comprises assisting in the control of the size and shape ofnanoparticles during the reduction of the metal compound. For example,if the amount of adsorptive substance is not sufficient to shield thegrowing nanoparticles completely, the formation of particles with a highaspect ratio such as, e.g., nanorods and/or nanowires and/or irregularlyshaped particles may be observed. Also, under otherwise identicalconditions, the average size of the formed nanoparticles will usuallydecrease with increasing molar ratio of adsorptive substance and metalcompound. It has been found that under otherwise identical conditions,the rapid mixing of the solution of the metal compound and the heatedsolution according to the process of the present invention allows toobtain substantially the same results with respect to the control of thesize, the size distribution and/or the shape of the particles as theknown method with its gradual dissolution/reaction of the metal compoundin the presence of the adsorptive compound, but at a (substantially)lower molar ratio of the adsorptive compound and the metal compound thanrequired in the known method. At any rate, the adsorptive substanceshould be present in at least the amount that is sufficient tosubstantially prevent an agglomeration of the nanoparticles. This amountis at least in part dependent on the size of the metal cores of theformed nanoparticles.

By way of non-limiting example, a batch of dry nanoparticles willusually require a minimum of surface passivation or surface coverage bythe adsorptive substance in order to be redispersible. A simple rule ofthumb is that the smaller the particle size the larger the surface areaand thus, the more adsorptive substance is required for completecoverage. Depending on the adsorptive substance, one can make somesimple assumptions regarding the thickness of a monolayer of adsorptivesubstance that is adsorbed on the surface of the metal cores of thenanoparticles. In addition, one may also assume that a minimum of onemonolayer of adsorptive substance around a metal core is needed to allowfor complete dispersibility of dry particles. Usually, not more thanabout 10 monolayers (and often not more than about 5 monolayers or evennot more than about 2 monolayers) of adsorptive substance are needed toredisperse and stabilize a metal nanoparticle in solution. With thissimple model one may make a rough estimate of the amount of adsorptivesubstance that is needed to (re)disperse metal nanoparticles of anysize. For example, for PVP as the adsorptive substance, one may assumethat the thickness of a monolayer thereof is about 1 nm. Based on thismodel and using the densities of Ag (10.5 g/cm³) and PVP (1.0 g/cm³) onecan calculate that for a PVP coated sphere-shaped silver core having adiameter of 20 nm the minimum amount of PVP needed to disperse a dryparticle is about 3.2% by weight (one monolayer). Preferably, not morethan 10 monolayers or 41% by weight of PVP will be present. Mostpreferably, about 4 to about 8 monolayers or about 14.8% to about 32.5%by weight of PVP will be used. For a 50 nm PVP-coated sphere-shaped Agcore, a minimum of about 1.3% by weight of PVP will be needed to coverthe nanoparticle completely with a monolayer. No more than about 14.8%by weight or 10 monolayers will usually be needed. Most preferably about5.3 to about 11.5% by weight of PVP is used (for about 4 to about 8monolayers).

If the adsorptive substance is or comprises a low molecular weightcompound (i.e., one or more low molecular weight compounds, collectivelyreferred to herein as a single compound), the molar ratio of the lowmolecular weight compound and the metal in the reaction mixture willoften be at least about 3:1, e.g., at least about 5:1, or at least about10:1. While there is no particular upper limit for this ratio, forpractical reasons and reasons of economic efficiency one will usuallyavoid a substantially higher amount of adsorptive substance than theamount that is needed for obtaining particles in the desired size rangeand/or for substantially preventing an agglomeration of thenanoparticles.

If the adsorptive substance is or comprises a polymer (i.e., one or morepolymers, collectively referred to herein as a single polymer), themolar ratio in the reaction mixture of the monomer units of the polymer(and preferably of only those monomer units that are capable of beingadsorbed on the nanoparticles), and the metal will often be at leastabout 3:1, e.g., at least about 5:1, at least about 6:1, at least about8:1, or at least about 10:1. However, for practical reasons (inparticular in view of the viscosity increasing effect of certainpolymers) and for reasons of economic efficiency (excess adsorptivesubstance, i.e., substance that will not be adsorbed may have to beremoved and discarded/recycled later) this ratio will usually be nothigher than about 100:1, e.g., not higher than about 80:1, not higherthan about 50:1, not higher than about 30:1, or not higher than about20:1.

Reaction Time and Temperature

The reaction between the metal compound and the polyol will usually takesome time (e.g., one or more hours) before a substantial percentage ofthe employed metal compound has been converted to metal nanoparticles.The reaction rate depends, inter alia, on the temperature at which themixed solutions are kept. It will usually be advantageous to heat themixed solutions to an elevated temperature (if they are not at thedesired temperature already) and to keep them at this temperature for asufficient period to convert at least a substantial portion of, andpreferably substantially the entire metal compound (e.g., at least about90%, or at least about 95% thereof) to metal nanoparticles. Thetemperature that is needed to achieve a desired degree of conversionwithin a predetermined period of time depends, inter alia, on thereactivities and concentrations of the reactants. Of course, thereaction temperature should not be so high as to cause a more thaninsignificant decomposition of the various components of the reactionmixture (e.g., of the adsorptive substance). Also, the temperature willusually be not significantly higher than the boiling point of thelowest-boiling component of the reaction mixture, although this is notmandatory, especially if the reaction mixture is kept under a higherthan atmospheric pressure, e.g., in an autoclave. In many cases, thereaction mixture will be heated to/kept at a temperature of at leastabout 80° C., e.g., at least about 90° C., at least about 100° C., atleast about 110° C., or at least about 120° C. On the other hand, itwill usually be advantageous for the temperature of the reaction mixtureto not exceed about 200° C., e.g., to not exceed about 180° C., to notexceed about 160° C., to not exceed about 150° C., or to not exceedabout 140° C.

Especially (but not only) in cases where the volume of the liquid phaseis kept relatively small relative to the amount of components dissolvedtherein and/or the reaction temperature is relatively close to theboiling point of the liquid phase or a component thereof, respectively,and no reflux is provided for, it may be advantageous to add additionalsolvent, in particular, a polyol to the reaction mixture.

One of skill in the art will understand that the process of the presentinvention can be carried out batch-wise, and also semi-continuously orcontinuously. By way of non-limiting example, in the case of acontinuous process, separate feeds of the solution of the metal compoundand the heated solution may be introduced continuously into a constanttemperature reactor. Reaction product is continuously withdrawn at thesame rate as the rate at which the two feed streams are introduced intothe reactor. Alternatively, the feed streams may be premixed before theyenter the reactor as a single feed stream. The required residence timein the reactor can be calculated on the basis of the reaction rate atthe selected temperature, the desired degree of conversion, etc.

Optional Further Processing

Once the desired degree of conversion of the metal compound is achievedthe reaction mixture is preferably cooled to room temperature. Coolingcan be accomplished in any suitable manner, e.g., by cooling thereaction vessel with a cooling medium such as, e.g., water (forcedcooling). Further, the reaction mixture may simply be allowed to cool toroom temperature in the ambient atmosphere.

Preferably after the cooling of the reaction mixture to room temperaturethe formed nanoparticles may be separated from the liquid phase of thereaction mixture. This can be accomplished, for example, in the variousways of separating a solid from a liquid that are known to those ofskill in the art. Non-limiting examples of suitable separation methodsinclude filtration, centrifugation, chromatographic methods,electrophoretic techniques, etc. Because the nanoparticles have a verysmall mass, do not substantially agglomerate and have adsorptivesubstance thereon that will usually interact with the components of theliquid phase, the nanoparticles will often not settle, i.e., separatefrom the liquid phase by themselves, at least not to a sufficient extentand/or within a desirably short period of time. A preferred method ofachieving a separation of the nanoparticles from the liquid phase of thereaction mixture comprises the addition of one or morenanoparticle-precipitating substances to the reaction mixture. Thesuitability of a substance for causing a precipitation of thenanoparticles will usually depend, inter alia, on the nature of theadsorptive substance. A non-limiting example of ananoparticle-precipitating substance includes a substance thatinterferes with the (electronic and/or steric) interaction between theadsorptive substance that is adsorbed on the surface of thenanoparticles and one or more components of the liquid phase. Apreferred example of such a nanoparticle-precipitating substance is asolvent in which the adsorptive substance is substantially insoluble orat least only poorly soluble. The nanoparticle-precipitating substanceis preferably substantially soluble in and/or miscible with the liquidphase of the reaction mixture, in particular, the polyol(s). Often thissubstance will comprise an aprotic solvent, preferably a polar aproticsolvent. The term “aprotic” characterizes a solvent that is not capableof releasing (dissociating into) protons. Non-limiting examples of suchsolvents include ethers (e.g., diethyl ether, tetrahydrofuran,tetrahydropyran, etc.), sulfonyl compounds and particularly, carbonylcompounds such as, e.g., ketones, esters and amides, especially ketones.Preferred ketones comprise from 3 to about 8 carbon atoms such as, e.g.,acetone, butanone, the pentanones, the hexanones, cyclopentanone andcyclohexanone. Of course, mixtures of aprotic solvents may be used aswell.

The nanoparticle-precipitating substance(s) will usually be employed inan amount which is sufficient to cause a precipitation of at least asubstantial portion of the nanoparticles that are present in thereaction mixture, e.g., at least about 90%, at least about 95%, or atleast about 98% of the nanoparticles.

While the addition of a nanoparticle-precipitating substance in asufficient quantity may result in a precipitation, the precipitationwill frequently be unsatisfactory, particularly in cases where thevolume of the liquid phase of the reaction mixture is substantial (e.g.,at least about 1 liter, at least about 2 liters, or at least about 3liters) and/or the concentration of the adsorptive substance and/or thenanoparticles in the liquid phase is relatively high. For example, theaddition of the nanoparticle-precipitating substance in a sufficientquantity to cause a precipitation of substantially all of thenanoparticles may frequently cause a concomitant precipitation of atleast a substantial portion of the excess (unbound) adsorptive substancethat is present in the reaction mixture. The precipitated adsorptivesubstance may form an oil which prevents or at least significantlyinterferes with (slows down) a settling of the nanoparticles, therebymaking the separation of the nanoparticles from the liquid phasedifficult, if not impossible.

It has been found that the required settling times can be shortenedand/or the formation of oily precipitates can be significantly reducedor completely eliminated if before and/or during and/or after theaddition of the nanoparticle-precipitating substance a protic solvent isadded to the reaction mixture. The term “protic” characterizes a solventthat is capable of releasing (dissociating into) protons. Preferably,the protic solvent comprises a hydroxyl-containing compound, inparticular, an alcohol such as, e.g., ethanol, propanol, butanol and thelike, and/or a polyol, e.g., any of the polyols that may be used as areactant in the process of the present invention. Water may also be usedas the (or a part of the) protic solvent. Preferably, the protic solventis or comprises one or more of the polyols that are present in thereaction mixture. Details regarding the addition of the protic solventmay be taken from U.S. Provisional Application Ser. No. 60/643,629entitled “Separation of Metal Nanoparticles,” the entire disclosurewhereof is expressly incorporated by reference herein.

According to a preferred aspect of the process of the present invention,the precipitated nanoparticles are isolated by removing the liquid phaseof the reaction mixture therefrom. Any process that is suitable forremoving a liquid from a solid can be used for this purpose.Non-limiting examples of such a process include decantation, filtration,centrifugation and any combinations thereof. Preferably, thenanoparticles are isolated by centrifugation (including, for example,continuous centrifugation), filtration (including ultrafiltration,diafiltration etc.) or a combination of two or more of these processes.

With respect to continuous centrifugation, this can be accomplished indifferent ways. For example, one may use a unit (centrifuge) which isoptimized for affording at least three different products in differentsections of the unit, for example, a supernatant in a top section,undesirably large particles in a bottom section and desired product(e.g., nanoparticles in the desired particle size range) in a middlesection. Each of these three products may be withdrawn continuously fromthe centrifuge while a fresh mixture for separation is continuouslyintroduced into the centrifuge. According to another alternative of thecontinuous centrifugation, two or more centrifuges may be arranged inseries, each of them being optimized for the removal of one kind ofseparation product, e.g., supernatant, undersized particles, particlesin the desired particle size range, oversized particles, etc.

Regarding the ultrafiltration/diafiltration of nanoparticles referencemay be made, for example, to U.S. Pat. Nos. 6,328,894, 5,879,715,6,245,494 and 6,811,885, the entire disclosures whereof are incorporatedby reference herein. Briefly, ultrafiltration and diafiltration use afiltration under pressure through a membrane which allows onlycomponents of a certain maximum size to pass therethrough. In thepresent case, the metal nanoparticles will be retained by the membranewhile preferably a major part or substantially all of the contaminants(e.g., dissolved inorganic matter, excess adsorptive substance, etc.)and the like will be able to pass through the membrane. Any size ofmembrane as well as any channel (pore) size thereof can be used as longas the process permits a preferably substantial removal of contaminantsand the like while retaining substantially all of the nanoparticles. Ina preferred aspect, the membrane may vibrate to substantially reduceclogging and/or to permit a higher permeate flow rate. Also, theultrafiltration/diafiltration may be pressure-driven (i.e., involvingpressing through the membrane) or vacuum-driven (i.e., involving suckingthrough the membrane). Membrane configurations include, but are notlimited to, flat sheet membranes, cross flow membranes, spiral woundtubes, or hollow fiber tubes. For example, a three compartmentthrough-flow cell comprising two membranes may be used. Non-limitingexamples of membrane materials include polymeric and ceramic materialssuch as, e.g., polysulfone, polyethersulfone, sulfonated polysulfone,polyamide, cellulose ester (e.g., cellulose acetate), and metal oxidessuch as the oxides of titanium, zirconium, silicon, aluminum andcombinations of two or more thereof. By way of non-limiting example, themembrane may have a molecular weight cutoff (MWCO) in the range of fromabout 10,000 to about 1,000,000, e.g., about 50,000, about 100,000,about 200,000 or about 500,000, and/or a pore size of from about 0.01 μmto about 1 μm (preferably at least about 0.02 μm and not higher thanabout 0.5 μm) and/or a lumen of from about 0.1 mm to about 5 mm(preferably at least about 2 mm and not more than about 3 mm).

Any type of ultrafiltration/diafiltration process may be used as long asthe process is capable of removing a substantial portion of thecontaminants and the like (e.g., at least about 70%, at least about 80%,at least about 90%, or at least about 95%) and in particular, that partof the adsorptive substance that is not adsorbed on the surface of thenanoparticles while retaining substantially all of the nanoparticles. Byway of non-limiting example, a cross-flow separation process may beused.

The nanoparticles that have been separated from the liquid phase arepreferably subjected to a washing operation to remove at least asubstantial portion of the impurities that may still be associatedtherewith such as, e.g., materials that are not adsorbed on the surfaceof the nanoparticles to any significant extent. For example, theseimpurities may include inorganic salts formed during the reduction ofthe metal compound, residual solvent(s) from the precipitation step andexcess adsorptive substance, i.e., adsorptive substance that is merelypresent as an impurity without being adsorbed on the nanoparticles. Thewashing liquid used for the washing operation preferably is or comprisesa solvent that is capable of dissolving the impurities associated withthe nanoparticles, in particular, excess adsorptive substance. By way ofnon-limiting example, the washing liquid may comprise water and/or anorganic solvent, for example, a polar organic solvent. One of skill inthe art will appreciate that the most desirable washing liquid(s) for aspecific case will to a large extent depend on the nature of theimpurities to be removed, e.g., polar vs. apolar, inorganic vs. organic,etc. In some cases it may be advantageous to use two or more differentwashing liquids (preferably successively or with a concentrationgradient) for obtaining the best results. Non-limiting examples of awashing liquids include liquids which comprise or consist of waterand/or one or more protic organic solvents such as, e.g., ahydroxyl-containing compound, preferably, an alcohol and/or a polyol.Illustrative and non-limiting examples of alcohols and polyols that maybe used for the washing operation include aliphatic alcohols having from1 to about 12 carbon atoms such as, e.g., methanol, ethanol, propanol,isopropanol, butanol, pentanol, cyclopentanol, hexanol, cyclohexanol,octanol, decanol, dodecanol and the like, and polyols having from 1 toabout 4 hydroxyl groups and from 2 to about 12 carbon atoms such as,e.g., ethylene glycol, propylene glycol, glycerol and the like. Apreferred solvent for use in the washing operation includes ethanol,which may be used alone or in combination with other solvents (e.g.,water). Of course, non-protic solvents may also be useful for thewashing operation. Non-limiting examples thereof include ketones suchas, e.g., acetone and butanone, ethers such as, e.g., diethylether andtetrahydrofuran, esters such as, e.g., ethyl acetate, amides such as,e.g., dimethylformamide and dimethylacetamide, sulfoxides such as, e.g.,dimethyl sulfoxide, and optionally halogenated hydrocarbons such as,e.g., hexane, cyclohexane, heptane, octane, petrol ether, methylenechloride, chloroform, toluene, the xylenes, etc. Combinations of two ormore of these solvents may, of course, also be used.

The washing operation may, for example, be carried out by dispersing theisolated crude nanoparticles obtained after, e.g., a filtration(including, e.g., a diafiltration/ultrafiltration) and/or centrifugationof the reaction mixture in a washing liquid, followed by a separation ofthe particles from the washing liquid by, e.g., filtration and/orcentrifugation. This process may optionally be repeated one or moretimes. The washed (purified) nanoparticles may thereafter be dried(e.g., under reduced pressure and/or at a temperature that does notadversely affect the adsorptive substance to any significant extent) andthereafter stored and/or shipped. Even after storage for extendedperiods the dry particles can be redispersed in a desired liquid to forma dispersion (e.g., a printing ink) that is substantially stable overseveral days or even weeks (for example, wherein not more than about20%, e.g., not more than about 10%, or not more than about 5% of thenanoparticles have settled after storing the dispersion at roomtemperature for at least one day, e.g., at least two days, or at leastone week).

In a preferred aspect of the present invention, the further processingof the nanoparticle containing reaction mixture obtained after carryingout the reduction of the metal compound may be carried out by usingsubstantially only ultrafiltration/diafiltration. In particular, the useof ultrafiltration/dialfiltration makes it possible to dispense with theaddition of a nanoparticle-precipitating substance to the reactionmixture and even allows combining the separation and washing operationsof the nanoparticles in a single operation. Further, at least the lastliquid that is added to the nanoparticles before theultrafiltration/diafiltration thereof is completed may be selected to bethe vehicle of a desired dispersion of the nanoparticles (for example,of a printing ink) or a component thereof, thereby making it possible toconvert the nanoparticle containing reaction mixture into the desirednanoparticle containing product in a single unit/operation. Also, one ormore additives may be incorporated in the washing liquid and/or theliquid that is intended to be the vehicle of the desired dispersion or acomponent thereof. For example, in order to keep the dissolution ofadsorbed adsorptive substance at a minimum it may be advantageous to addsome adsorptive substance to, e.g., the washing liquid. Also, one ormore additives whose presence may be desirable in the final nanoparticlecontaining product (e.g., a printing ink) may be incorporated into theliquid used in the final step(s) of a diafiltration operation (such as,e.g., adhesion promoters, humectants, etc.).

By way of non-limiting example, the diafiltration/ultrafiltration may becarried out by placing the nanoparticle containing reaction mixture in adiafiltration unit and concentrating the reaction mixture therein to apredetermined fraction of the original volume by pressing (applicationof pressure) or drawing (application of vacuum) the reaction mixturethrough one or more ultrafiltration membranes of suitable MWCO/poresize. Thereafter, a first liquid that is capable of dissolvingimpurities and contaminants present in the reaction mixture (inparticular, excess adsorptive substance) may be added to theconcentrated reaction mixture (e.g., in an amount sufficient to restorethe originally employed volume of the reaction mixture) and theresulting mixture may be concentrated in the same way as the originallyemployed reaction mixture. A second liquid which is capable ofdissolving impurities and contaminants and which may be the same as ordifferent from the first liquid may be added to the resultingconcentrate and the resulting mixture may be concentrated again. Thisprocess may be repeated as often as necessary with a third, fourth, etc.liquid. Alternatively, before concentrating the original reactionmixture a predetermined amount of the first liquid may be added theretoand the resulting mixture may be concentrated, e.g., until the originalvolume of the reaction mixture is reached again. Then the second liquidmay be added and a second concentration operation may be carried out,etc. Of course, any combination of the two alternatives described abovemay be used as well. For example, the original reaction mixture may beconcentrated first and then the first liquid may be added in an amountwhich results in a volume of the resultant mixture which exceeds thevolume of the original reaction mixture, whereafter the resultantmixture may be concentrated to the volume of the original reactionmixture, whereafter a second liquid may be added, etc. At the end ofeach of these alternative ways of isolating/purifying the metalnanoparticles by ultrafiltration/diafiltration the liquid may be removedpartially or completely by ultrafiltration, leaving behind the purifiedsubstantially non-agglomerated metal nanoparticles with the adsorptivesubstance thereon, or a concentrated and stable dispersion thereof. Thenanoparticles may then optionally be dried to form a powder batch of drynanoparticles. Alternatively, the liquids that are used for thediafiltration operation may be selected such that at least at the end ofthe diafiltration operation the purified nanoparticles are combined witha liquid which is the vehicle or at least a part of the vehicle of adesired dispersion of the metal nanoparticles (e.g., a printing ink).The liquids which may be used for carrying out thediafiltration/ultrafiltration include those which have been mentionedabove in the context of the separation of the nanoparticles from theliquid phase and the washing of the separated nanoparticles.

It is to be appreciated that the use of ultrafiltration/diafiltration isadvantageous not only for the separation and/or purification of metalnanoparticles that have been produced by the process of the presentinvention but is a procedure which is of general applicability forseparating inorganic nanoparticles and, in particular, metalnanoparticles that have a polymeric substance (e.g., ananti-agglomeration substance as used in the process of the presentinvention) adsorbed on their surface from a liquid which comprises adissolved polymeric substance (either the same as or different from theadsorbed polymeric substance) and for purifying (e.g., washing) suchnanoparticles and for formulating such separated/purified nanoparticlesinto a desired product (e.g., a dispersion).

Metal Nanoparticles

Due to the particular way of combining the metal compound and the polyolaccording to the process of the present invention, it is possible tocontrol the size, the size distribution and/or the shape of thenanoparticles even on a large scale. For example, particles whichexhibit a high degree of uniformity in size and/or shape may be producedby the process of the present invention. For example, the process of thepresent invention is capable of affording particles with a substantiallyspherical shape. In one aspect of the present invention, at least about90%, e.g., at least about 95% of the nanoparticles formed by the processof the present invention may be of a substantially spherical shape. Inanother aspect, the particles may be substantially free of micron-sizeparticles (i.e., particles having a size of about 1 μm or above). Evenmore preferably, the nanoparticles may be substantially free ofparticles having a size (=largest dimension, e.g., diameter in the caseof substantially spherical particles) of more than about 500 nm, e.g.,of more than about 200 nm, or of more than about 100 nm. In this regard,it is to be understood that whenever the size and/or dimensions of thenanoparticles are referred to herein and in the appended claims, thissize and these dimensions refer to the nanoparticles without adsorptivesubstance thereon, i.e., the metal cores of the nanoparticles. Dependingon the type and amount of adsorptive substance, an entire nanoparticle,i.e., a nanoparticle which has the adsorptive substance thereon, may besignificantly larger than the metal core thereof. Also, the term“nanoparticle” as used herein and in the appended claims encompassesparticles having a size/largest dimension of the metal cores thereof ofup to about 900 nm, preferably of up to about 500 nm. By way ofnon-limiting example, not more than about 5%, e.g., not more than about2%, not more than about 1%, or not more than about 0.5% of the particlesthat are formed by the process of the present invention may be particleswhose largest dimension (e.g., diameter) is larger than about 200 nm,e.g., larger than about 150 nm, or larger than about 100 nm. In aparticularly preferred aspect, at least about 95% of the nanoparticlesmay have a size of not larger than about 80 nm and/or at least about 80%of the nanoparticles may have a size of from about 30 nm to about 70 nm.

In another aspect, the nanoparticles formed by the process of thepresent invention may have an average particle size (expressed as numberaverage) of at least about 10 nm, e.g., at least about 20 nm, or atleast about 30 nm, but preferably not higher than about 80 nm, e.g., nothigher than about 70 nm, not higher than about 60 nm, or not higher thanabout 50 nm. The average particle sizes and particle size distributionsreferred to herein may be measured by conventional methods such as,e.g., by scanning electron microscopy (SEM) or tunneling electronmicroscopy (TEM) and refer to the metal cores.

In yet another aspect of the process of the present invention, at leastabout 80 volume percent, e.g., at least about 90 volume percent of thenanoparticles formed by the process of the present invention may be notlarger than about 2 times, e.g., not larger than about 1.5 times theaverage particle size (volume average).

The reduction process of the present invention and the optional furtherprocessing of the reaction mixture obtained thereby are capable ofaffording large, commercially useful quantities of substantiallynon-agglomerated, dispersed or redispersable metal nanoparticles in asingle run. For example, in batch-wise operation the process of thepresent invention can be carried out on a scale at which at least about30 g, e.g., at least about 40 g, at least about 50 g, or at least about60 g of substantially non-agglomerated, dispersed or redispersable metal(e.g., silver) nanoparticles (expressed as pure metal without adsorptivesubstance) are produced in a single run. In a preferred aspect, a singlerun will afford at least about 100 g, at least about 200 g, or at leastabout 500 g of substantially non-agglomerated, dispersed orredispersable metal nanoparticles.

Further, in one aspect of the present invention a concentrated batch ofmetal nanoparticles—a so-called “masterbatch”—may be produced which maybe a liquid or solid at room temperature and comprises a highconcentration of metal nanoparticles and may be stored for an extendedperiod of time and subsequently redispersed by adding solvents and/ordiluents. By way of non-limiting example, the masterbatch may compriseadsorptive substance and metal nanoparticles alone, or the masterbatchmay comprise metal nanoparticles, adsorptive substance andsolvents/diluents. In another embodiment of this invention, aconcentrated metal nanoparticle batch may be further concentrated byremoving the solvent to produce a batch of dry metal nanoparticlescomprised primarily of metal nanoparticles and adsorptive substance,which batch can be substantially completely redispersed to form a stabledispersion (e.g., a desired printing ink) on adding a suitable solventor diluent liquid. Due to, for example, the reduced volume thereof, adry masterbatch is particularly advantageous for shipping and storageover an extended period of time. In this regard, it is to be appreciatedthat the adsorptive substance on the surface of the metal nanoparticleswill usually not only substantially prevent an (irreversible)agglomeration of the nanoparticles, but also increase the shelf life ofthe nanoparticles and in particular, of dry nanoparticles by shieldingto at least some extent the surfaces of the metal cores of thesenanoparticles from an attack by oxygen (oxidation), heat, harmfulradiation (e.g., UV rays) and the like.

Preferred Aspects

A preferred aspect of the process of the present invention comprises theproduction of silver nanoparticles by the rapid mixing a solution of atleast about 0.1 mole of a silver compound with a heated solution of apolyol and a vinyl pyrrolidone polymer. In the following descriptionthis aspect will be discussed with respect to particularly advantageousfeatures thereof.

Non-limiting examples of silver compounds for use in the preferredaspect of the process of the present invention include silver nitrate,silver acetate, silver trifluoroacetate and silver oxide. Particularlypreferred is silver nitrate. Preferably, the silver compound comprises asingle silver compound, although mixtures of different silver compoundsand mixtures of one or more silver compounds with one or more compoundsof other metals may be used as well.

The polyol(s) used in combination with the silver compound will usuallyhave from 2 to about 4 hydroxy groups and/or from 2 to about 6 carbonatoms such as, e.g., ethylene glycol, diethylene glycol, triethyleneglycol, propylene glycol, dipropylene glycol, 1,3-propanediol,1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol,glycerol, triethanolamine and trimethylolpropane. Usually, the polyol(s)will include on or more glycols, preferably at least one glycol havingfrom 2 to about 4 carbon atoms. Preferred examples of such glycolsinclude ethylene glycol and propylene glycol. Particularly preferred isethylene glycol.

The vinyl pyrrolidone polymer for use in combination with the silvercompound may be a vinyl pyrrolidone homopolymer (PVP) and/or a vinylpyrrolidone copolymer. Preferably, the polymer is or at least comprisesa vinyl pyrrolidone homopolymer.

The vinyl pyrrolidone polymer (and particularly, the vinyl pyrrolidonehomopolymer) will usually have a weight average molecular weight of upto about 60,000, e.g., up to about 50,000, up to about 40,000, or up toabout 30,000, and preferably, of not less than about 3,000, e.g., notless than about 4,000, not less than about 5,000, not less than about8,000, or not less than about 10,000. An example of a particularlypreferred vinyl pyrrolidone polymer is a vinyl pyrrolidone homopolymer(=PVP) having a weight average molecular weight of about 10,000.

The solution of the silver compound preferably comprises one or morepolyols. Even more preferably, at least one of the one or more polyolsis identical with at least one of the one or more polyols present in theheated solution. Most preferably, both the solution of the silvercompound and the heated solution comprise ethylene glycol, which may bethe only polyol used in the process of the present invention.

In another aspect of the preferred process of the present invention, nosolvent other than the polyol(s) may be present in the solution of thesilver compound and/or in the heated solution. Preferably, both thesolution of the metal compound and the heated solution will comprise oneor more polyols (e.g., ethylene glycol and/or propylene glycol) as theonly liquid component.

In yet another aspect of the preferred process of the present invention,the heated solution may be at a temperature of not higher than about150° C., preferably not higher than about 130° C., and not lower thanabout 80° C., preferably, not lower than about 100° C. A particularlypreferred temperature of the heated solution is about 120° C.

In a still further aspect of the preferred process of the presentinvention, the temperature of the solution of the silver compound may benot higher than about 40° C., e.g., not higher than about 30° C., or nothigher than about 25° C.

In another aspect of the preferred process of the present invention, therapid mixing may comprise combining the solutions within not more thanabout 30 seconds, preferably with agitation such as, e.g., stirring,shaking and/or sonication. Preferably, the solutions are combined by aone-shot addition of one of the solutions to the other solution, inparticular, by adding the solution of the silver compound to the heatedsolution.

In another aspect of the preferred process of the present invention, thesolution of the silver compound may comprise at least about 0.5 mole ofthe silver compound, e.g., at least about 0.75 mole of the silvercompound, or at least about 1 mole of the silver compound.

In yet another aspect of the preferred process of the present invention,the combined volume of the solution of the silver compound and theheated solution per one mole of the silver compound may not be largerthan about 5 liters, e.g., not larger than about 4 liters, and may oftenbe not smaller than about 3 liters, e.g., not smaller than about 2liters per one mole the of silver compound.

In yet another aspect of the preferred process of the present invention,the volume ratio of the heated solution and the solution of the silvercompound may be not higher than about 8:1, e.g., not higher than about7:1, or not higher than about 6:1, and not lower than about 2:1, e.g.,not lower than about 3:1, or not lower than about 4:1. A particularlypreferred volume ratio is about 5:1. By way of non-limiting example, thesolution of the silver compound (e.g., silver nitrate) may compriseabout 1.5 mole of silver compound per liter of solvent (e.g., ethyleneglycol) and the ratio of the heated solution and the solution of thesilver compound may be about 5:1.

In a still further aspect of the preferred process of the presentinvention, the molar ratio of the vinyl pyrrolidone units in the vinylpyrrolidone polymer (which preferably comprises or consists of a vinylpyrrolidone homopolymer) and the silver compound will preferably be notlower than about 3:1, e.g., not lower than about 6:1, or not lower thanabout 10:1, and will preferably be not higher than about 100:1, e.g.,not higher than about 50:1, not higher than about 20:1, or not higherthan about 15:1. By way of non-limiting example, the molar ratio ofvinyl pyrrolidone units of a PVP and the silver compound may be about12:1.

In one aspect, the preferred process of the present invention mayfurther comprise keeping the mixed solutions at a temperature of atleast about 100° C., e.g., at least about 110° C., for a sufficientperiod to convert a substantial portion (preferably at least about 90%,and more preferably at least about 95%, and most preferably at leastabout 98%) of the silver compound to silver nanoparticles. In apreferred aspect, the mixed solutions are heated at a temperature ofabout 120° C. for about 1 hour and/or until substantially all of silvercompound is converted to nanoparticles.

In another aspect, the preferred process of the present invention mayfurther comprise the addition of a polyol to the mixed solutions. Thepolyol preferably comprises or is a glycol, preferably a glycol that isalready present in the mixed solutions. The amount of glycol willpreferably be at least about the amount that compensates any losses ofpolyol due to evaporation during the heating of the mixed solutions. Thepolyol may be added, for example, continuously, in two or more portionsor in one shot.

In another aspect, the preferred process of the present invention mayfurther comprise allowing the heated mixture to cool to roomtemperature.

In yet another aspect, the preferred process of the present inventionmay further comprise a precipitation of formed silver nanoparticles.This precipitation preferably comprises the addition of a sufficientamount of a nanoparticle-precipitating liquid that is miscible with theone or more polyols that are present in the reaction mixture toprecipitate at least a substantial portion (preferably at least about90%) of the silver nanoparticles.

The nanoparticle-precipitating liquid will usually comprise a polaraprotic solvent such as, e.g., a ketone. A non-limiting example of aketone is acetone.

In a preferred aspect of the precipitation of the silver nanoparticles,before and/or during and/or after the addition of thenanoparticle-precipitating liquid a polyol is added in a sufficientamount to improve the precipitation of the nanoparticles and/or theseparation thereof from the liquid phase. This polyol may be or maycomprise a polyol that is already present in the reaction mixture. Forexample, the polyol may comprise a glycol such as ethylene glycol and/orpropylene glycol. By way of non-limiting example, thenanoparticle-precipitating liquid may comprise acetone and the polyolmay comprise ethylene glycol.

In yet another aspect, the preferred process of the present inventionmay further comprise an isolation of the (precipitated) silvernanoparticles. This isolation can be accomplished by any means that aresuitable for removing a liquid from a solid. However, the separationpreferably includes a centrifugation (including, e.g., a continuouscentrifugation) and/or an ultrafiltration and/or a diafiltration.

In a still further aspect, the preferred process of the presentinvention may further comprise subjecting the isolated silvernanoparticles to a washing operation with a liquid that is capable ofdissolving the vinyl pyrrolidone polymer. Preferably, the liquidcomprises an aliphatic alcohol such as, e.g., ethanol and/or water. Thewashing operation will usually remove at least a major part of theexcess vinyl pyrrolidone polymer that is not adsorbed on thenanoparticles (i.e., is present merely as a contaminant). While thepresence of major quantities of excess polymer may be acceptable formany applications for which the silver nanoparticles are currentlyintended, there are also applications in which the presence ofsignificant amounts of excess polymer should be avoided.

In particular, the silver nanoparticles of the present invention may beused in the formulation of printing inks for various purposes and forvarious printing techniques such as, e.g., ink-jet printing, screenprinting, intaglio printing, roll printing, lithographic printing andgravure printing. Major fields of application for these inks includeelectronics (e.g., for the printing of electrically conductive featuresand the like by, for example, ink-jet printing), graphics (e.g.,decorative purposes), security features and the like. In many of theseapplications, the excess polymer is either acceptable or can be removedby, e.g., thermal means, for example, by heating the deposited ink to atemperature above the decomposition temperature of the polymer (which atthe same time will remove the polymer that is adsorbed on thenanoparticles). The removal of excess polymer by the application ofthermal energy is particularly suitable for applications which involveheat-resistant materials (substrates) such as, e.g., glass, ceramic,metal and heat-resistant polymers.

Especially in the field of electronics and, in particular inapplications which involve temperature-sensitive substrates (e.g.,polymers, cellulose-based materials etc.) the presence of significantamounts of excess polymer is undesirable because on the one hand theexcess polymer may have a substantial adverse effect on the conductivityof the electrically conductive features printed on the substrates and onthe other hand the removal of the excess polymer by thermal means willusually not be possible without severely damaging or destroying thesubstrate. Accordingly, in these cases the silver nanoparticles shouldbe substantially free of excess polymer. While even the nanoparticleswhich are free of excess polymer still have vinyl pyrrolidone polymeradsorbed on their surfaces it has unexpectedly been found that with theadsorbed polymer alone substantial electric conductivity can be achievedat sintering temperatures of the deposited ink which are as low as about100° C. Without wishing to be bound by any theory, it is assumed thatalready at these low sintering temperatures the adsorbed vinylpyrrolidone polymer becomes mobile enough to at least partially andsufficiently expose the silver cores of the individual nanoparticles toallow a direct contact between (and sintering of) these silver cores,whereby electrical conductivity is established. Details regarding theformulation and properties of printable inks comprising vinylpyrrolidone polymers are disclosed in U.S. Provisional Application Ser.No. 60/643,577 entitled “Metal Nanoparticle Compositions,” the entiredisclosure whereof is incorporated by reference herein.

In a further aspect of the preferred process of the present invention,at least about 90%, e.g., at least about 95% of the silver nanoparticlesmay be of a substantially spherical shape, and/or at least about 90%,preferably at least about 95% of the nanoparticles may have a diameterof not more than about 80 nm and/or at least about 90% of thenanoparticles may have a diameter of from about 10 nm to about 70 nmand/or at least about 80 volume percent of the nanoparticles may be notlarger than about 1.5 times the average particle size. Preferably, thesilver nanoparticles will be substantially free of micron-size (andlarger) particles. Even more preferably, they will be substantially freeof particles having a size of more than about 200 nm, e.g., of more thanabout 150 nm.

EXAMPLES

The present invention will be further illustrated by the followingnon-limiting examples.

Example 1

In a mixing tank a solution of 1000 g of PVP (M.W. 10,000, Aldrich) in2.5 L of ethylene glycol is prepared and heated to 120° C. In a secondmixing tank, 125 g of silver nitrate is dissolved in 500 ml of ethyleneglycol at 25° C. These two solutions are rapidly combined (within about5 seconds) in a reactor, in which the combined solutions (immediatelyafter combination at a temperature of about 114° C.) are stirred at 120°C. for about 1 hour. The resultant reaction mixture is allowed to coolto room temperature and about 0.25 L of ethylene glycol is added theretoto replace evaporated ethylene glycol. This mixture is stirred at highspeed for about 30 minutes to resuspend any particles that have settledduring the reaction. The resultant mixture is transferred to a mixingtank where 12 L of acetone and about 1 L of ethylene glycol are added.The resultant mixture is stirred thoroughly and then transferred to acentrifuge where it is centrifuged for about 20 minutes at 1,500 g toseparate the silver nanoparticles from the liquid phase. This affords 70g of nanoparticles which have PVP adsorbed thereon. The particles aresubsequently suspended in 2,000 ml of ethanol to remove, inter alia,excess PVP, i.e., PVP that is not adsorbed on the nanoparticles but ispresent merely as a contaminant. At this point, the ethanol suspensionof particles is preferably filtered through a 1.5 μm nylon filter, thusfiltering out particles that are larger than 1.5 μm. The filtrate issubsequently centrifuged and the resulting cake is dried in a vacuumoven at about 35° C. and about 10-2 torr to afford dry nanoparticles.These nanoparticles exhibit a PVP content of about 4 to about 8 weightpercent, depending on the time the nanoparticles have been in contactwith the ethanol.

Alternatively, the ethanol suspension may be centrifuged without firstfiltering through a 1.5 μm nylon filter. The resultant filter cake ofnanoparticles is dried in a vacuum oven at about 35° C. and about 10⁻²torr to afford dry nanoparticles. These nanoparticles, like theparticles obtained after filtering through a 1.5 μm filter, exhibit aPVP content of about 4 to about 8 weight percent, depending on the timethe nanoparticles have been in contact with the ethanol.

ICP (inductively coupled plasma) data indicates that the longer theparticles are in contact with the ethanol, the more of the acetone andethylene glycol present in the PVP matrix is displaced by ethanol,resulting in particles with an increasingly higher silver content.

It is believed that several characteristics of the reagents that areused in the process described above may ultimately affect the particlesize distribution (PSD) of the nanoparticles produced by the process. Atleast four such characteristics are the water content of the PVP, the2-pyrrolidone content of the PVP (PVP inherently contains2-pyrrolidone), the formic acid content of the PVP and the water contentof the ethylene glycol used.

Although the reasons are not entirely clear at this time, it has beenobserved that the water content of the PVP may have an effect on the PSDof the nanoparticles produced by the process described above. PVP is ahygroscopic substance that typically has a residual amount of watercontained therein. In some cases, the presence of water in the PVP leadsto the production of nanoparticles with a desirable PSD. In other cases,however, when the water content in the PVP is reduced, the same resultis observed. Thus, it is not entirely clear if it is preferable to havePVP where the water content has been reduced or PVP where the residualwater content is not reduced. In one embodiment, the water content ofthe PVP is about 1-10% by weight, or about 1-8% by weight, or about2-10% by weight or preferably about 2-7% by weight. The water content ofthe PVP may be reduced by heating the PVP at about 70-80° C. overnight(e.g., 8-14 hours). The PVP may optionally be stirred while it isheated. In addition, the heating may be optionally performed under aninert atmosphere (e.g., argon and nitrogen) or in vacuo.

Although the reasons are not entirely clear at this time, it has alsobeen observed that the presence of 2-pyrrolidone in the PVP may have aneffect on the PSD of the nanoparticles produced by the process describedabove. 2-pyrrolidone is a contaminant comprised in the PVP that resultsfrom the synthesis of PVP. In one embodiment, the 2-pyrrolidone contentof the PVP is about 1-10% by weight, or about 1-8% by weight, or about1-5% by weight or preferably about 5% by weight.

In addition, although the reasons are not entirely clear at this time,it has also been observed that the presence of formic acid in the PVPmay have an effect on the PSD of the nanoparticles produced by theprocess described above. Formic acid is a contaminant that may becomprised in the PVP that results from the synthesis of PVP. In somecases, the presence of formic acid in the PVP leads to the production ofnanoparticles with a desirable PSD. In other cases, however, when theformic acid content in the PVP is reduced, the same result is observed.Thus, it is not clear if it is preferable to have PVP where the formicacid content has been reduced or PVP where the formic acid content isnot reduced. In one embodiment, the formic acid content of the PVP isabout 0-2% by weight, or about 0-1% by weight, or about 0-0.8% by weightor preferably about 0.3% by weight. The formic acid content of the PVPmay be reduced by heating the PVP at about 70-80° C. overnight (e.g.,8-14 hours). The PVP may optionally be stirred while it is heated. Inaddition, the heating may be optionally performed under an inertatmosphere (e.g., argon and nitrogen) or in vacuo.

Finally, although the reasons are not entirely clear at this time, ithas also been observed that the presence of water in the ethylene glycolmay have an effect on the PSD of the nanoparticles produced by theprocess described above. It is possible that some water must be presentin the ethylene glycol in order to produce nanoparticles with adesirable PSD. Without being bound by any particular theory, it ispossible that the presence of water content in ethylene glycolinfluences the equilibrium between ethylene glycol and its decompositionproduct, acetaldehyde, according to the reaction shown below:

Even though it is possible that it is produced in an extremely smallquantity (i.e., the equilibrium favors ethylene glycol), theacetaldehyde produced, in turn, acts as a reducing agent for the silvernitrate, thereby effecting the reduction of silver ions in silvernitrate to silver metal. The silver metal, in turn, can precipitate outof the ethylene glycol solution prior to mixing the silvernitrate/ethylene glycol solution with the PVP/ethylene glycol solution.It is possible that the precipitation of silver metal in the silvernitrate/ethylene glycol solution, prior to mixing with the PVP/ethyleneglycol solution, could lead to the production of nanoparticles once thesolutions are mixed, where the nanoparticles have an undesirable PSD(e.g., a polydisperse powder fraction with an increased amount of largeparticles and agglomerates). In one embodiment, the water content of theethylene glycol is about 0.01-0.10% by weight, or about 0.01-0.05% byweight, or about 0.01-0.04% by weight or preferably about 0.04% byweight.

Example 2 Effect of PVP Molecular Weight on Resistivity and CuringTemperature

Silver nanoparticles can be synthesized using 58,000 g/mol PVP using aprocess substantially the same as the process used to synthesize silvernanoparticles using 10,000 g/mol PVP by mixing a PVP/ethylene glycolsolution with a silver nitrate/ethylene glycol solution. Once thereaction is complete, 250 mL of ethylene glycol is mixed into thereaction solution after cooling down. The resulting solution is equallydivided into five 4-L Nalgene® bottles. 200 mL of ethylene glycol and 3L of acetone are added to each plastic bottle. The bottles are sealedwith lids and shaken up to create a black precipitate suspension in thesolvent mixture. The suspensions are transferred to centrifuge bottlesand centrifuged at 2,200 RPM for 10 minutes. The clear, light orangesupernatants are discarded to leave behind a reflective, silver cake.More black suspension is added to the same bottles and the separationsteps are repeated until all of the silver nanoparticles are caked inthe bottom of the centrifuge bottles. 600 mL of ethanol are added toeach centrifuge bottle to remove, inter alia, excess PVP. At this point,the ethanol suspension of particles is preferably filtered through a 1.5μm nylon filter, thus filtering out particles that are larger than 1.5μm. The filtrate is subsequently centrifuged and the resulting cake isdried in vacuo for 2-3 hours.

Alternatively, the ethanol suspension may be centrifuged withoutfiltering through a 1.5 μm nylon filter. In this case, the bottles arecentrifuged at 2,200 RPM overnight, and the clear, dark orangesupernatants are discarded to leave behind a highly reflective,bluish-silver cake. The silver nanoparticles are then dried in vacuo for2-3 hours to form gold-colored granules.

Silver nanoparticle inks are spin coated onto glass and tested toascertain the electrical properties. Liquid crystal display (LCD) glassslides are cut into 2″ squares and given 5 minutes of UV-Ozonetreatment. This is followed by wiping the slides with a lint-free clothwetted with acetone and then another lint-free cloth wetted by denaturedethanol. In succession, each slide is secured to a 2″ vacuum chuck onthe spin coater. The slide is spun at 500 RPM for 10 seconds while 0.8mL of silver ink is dispensed using a micropipette, after which theslide is spun at 1,000 RPM for 20 seconds. After spin coating, slidesare placed in a pre-heated Despatch oven set to either 100 or 120° C.for 60 minutes to cure the silver ink films. Then the silver-coatedslides are removed and allowed to cool. Each slide is placed on thefour-point probe station, and five resistance measurements are takennear the center of the silver films. After the measurements are taken, a1-2 mm wide, 0.5-1 cm long scratch is made in the center of each filmusing a stainless steel scalpel. When making the scratch, enoughpressure is applied to remove the ink layer but not enough tosignificantly scratch the glass substrate. Then the layer thickness ismeasured across five locations along the scratch, using the scratch as areference, on the Zygo profilometer. Finally, the resistance andthickness measurements are used to calculate the resistivity for eachsilver film.

The electrical testing of six silver inks formulated from differentsilver nanoparticles made with the 10,000 g/mol PVP it was observed thatthe average resistivity of the samples cured at 100° C. is 101× bulksilver over a range of 22.1-234× bulk silver. For curing at 120° C., theaverage resistivity of the samples is 17.8× bulk silver over a range of14.3-24.2× bulk silver. Unexpectedly, for the ink sample made withsilver nanoparticles formed in the presence of 58,000 g/mol PVP, theaverage resistivity is 12.8× bulk silver when cured at 100° C. and 7.20×bulk silver when cured at 120° C. It is evident from the data givenabove that, relative to inks that employ 10,000 g/mol PVP, the ink thatemploys 58,000 g/mol PVP results in superior electrical performance at arelatively low curing temperature. The ability of achieving suchsuperior electrical performance at relatively low curing temperatureswith inks comprising 58,000 g/mol PVP facilitates printing such inks onflexible substrates.

Example 3

To demonstrate the effect of the rate of addition and the temperature ofthe silver nitrate solution, a reaction of the type described in Example1 was conducted with different addition rates and temperatures of thesolutions. The results are summarized in Table 1 below.

TABLE 1 Temperature of AgNO₃/ Temperature of PVP/ Experiment ethyleneglycol ethylene glycol No. solution [° C.] solution [° C.] AdditionComments 1 RT 120 Half&half/20 min Larger particles, good sizedistribution, long needles (up to 100 μm) 2 RT 120 Drop-wise/20 min Nosize and shape control 3 120 120 Half&half/20 min Some large chunks,broad size distribution 4 120 120 Drop-wise/20 min No control, largechunks 5 120 120 One-shot Good control of size and shape, narrow sizedistribution 6 RT RT One-shot Good control of size and shape, relativelybroad distribution 7 RT 120 One-shot Good control of size and shape,narrow size distribution; slightly better than Exp. 5 RT = roomtemperature

From the results summarized in Table 1 it can be seen that the drop-wiseaddition of the silver nitrate solution to the PVP solution (which issimilar to dissolving solid silver nitrate in the PVP solution) does notallow any control of the size and shape of the silver nanoparticles.Adding the silver nitrate solution in two portions 20 minutes apartaffords a somewhat better but still unsatisfactory result. The one-shotaddition of the silver nitrate solution to the PVP solution at roomtemperature (Experiment 6) results in a relatively broad particle sizedistribution, while it is satisfactory in terms of particle size andshape. The one-shot addition of a silver nitrate solution to a 120° C.PVP solution affords a good control of particle size and shape and anarrow size distribution, with slightly better results in the case of asilver nitrate solution at room temperature compared to a solution at120° C.

Example 4

To 4 L of a particle suspension composed of 1.0 kg of unbound PVP, 3 Lof ethylene glycol and about 71 g of silver nanoparticles having PVPadsorbed thereon (prepared according to a process similar to thatdescribed in Example 1) is added 12 L of acetone. The resultant mixtureis subjected to centrifugation at 1,500 g. An oil suspension is formedduring centrifugation. The particle oil suspension is difficult to breakand complete separation of the particles from the liquid phase cannot beaccomplished.

Example 5

Example 3 is repeated, except that 1.25 L of ethylene glycol is addedbefore carrying out the centrifugation. The extra ethylene glycolprevents the formation of an oil. The particles form a cake and completeseparation of the particles from the liquid phase can be accomplished.

Example 6

To 1 L of a particle suspension composed of 250 g of unbound PVP, 750 mLof ethylene glycol and about 20 g of silver nanoparticles having PVPadsorbed thereon (prepared according to a process similar to thatdescribed in Example 1) is added 1 L of deionized water and theresultant mixture is subjected to a diafiltration in a tangentialcross-flow manner. (membrane made of polysulfone, pore size 50 nm,surface area 615 cm², Spectrum Laboratories, Inc.; applied pressureabout 10 psi gauge to about 20 psi gauge). The diafiltration is carriedout in concentration mode and is performed until the retentate has avolume of about 100 mL. To the retentate is added 1 L of deionized waterand diafiltration is again performed in concentration mode until thevolume of the retentate is about 100 mL. To the resultant secondretentate is added 250 mL of a mixture of 40% by weight of ethyleneglycol, 25% by weight of glycerol and 35% by weight of ethanol anddiafiltration is performed in concentration mode until the volume of theretentate is about 100 mL. To the resultant third retentate is added 103mL of a mixture of 40% by weight of ethylene glycol, 25% by weight ofglycerol and 35% by weight of ethanol and diafiltration is performed inconcentration mode until the volume of the retentate is about 100 mL.The resultant fourth retentate is passed through syringe filters(diameter 30 mm, pore size 1.5 μm) to prepare an ink for ink-jetprinting. This ink has properties similar to those of an ink formulatedfrom dry silver nanoparticles which have been synthesized in the samemanner as the particles of the initial suspension.

Example 7

To 1 L of a particle suspension composed of 250 g of unbound PVP, 750 mLof ethylene glycol and about 20 g of silver nanoparticles having PVPadsorbed thereon (prepared according to a process similar to thatdescribed in Example 1) is added 2 L of deionized water and theresultant mixture is subjected to a diafiltration in a tangentialcross-flow manner. The diafiltration is carried out in concentrationmode and is performed until the retentate has a volume of about 100 mL.To the retentate is added 1 L of deionized water and diafiltration isperformed in concentration mode until the volume of the retentate isabout 100 mL. To the resultant second retentate is added 500 mL ofethanol and diafiltration is performed in concentration mode until thevolume of the retentate is about 100 mL. The resultant third retentateis filled in a container with a loose fitting top and placed in a vacuumoven that is maintained at 35° C. A vacuum is applied on the oven andcontinuously drawn, maintaining the pressure at about 845 mbar until theparticles are dry. The dried particles are analyzed by TGA(thermogravimetric analysis) and ICP. TGA indicates that the particlescontain about 90% by weight of silver. ICP indicates the followingcomposition (in % by weight): Ag 86.54, N 0.85, C 5.48, H 0.85. Thedried particles are formulated into an ink for ink-jet printing.

Example 8

To 1 L of a particle suspension composed of 250 g of unbound PVP, 750 mLof ethylene glycol and about 20 g of silver nanoparticles having PVPadsorbed thereon (prepared according to a process similar to thatdescribed in Example 1) is added 2 L of deionized water and theresultant mixture is subjected to a diafiltration in a tangentialcross-flow manner. The diafiltration is carried out in concentrationmode and is performed until the retentate has a volume of about 100 mL.To the retentate is added 2 L of deionized water and diafiltration isperformed in concentration mode until the volume of the retentate isabout 100 mL. To the resultant second retentate is added 2 L ofdeionized water and diafiltration is performed in concentration modeuntil the volume of the retentate is about 100 mL. To the resultantthird retentate is added 1 L of ethanol and diafiltration is performedin concentration mode until the volume of the retentate is about 100 mL.The resultant fourth retentate is filled in a container with a loosefitting top and placed in a vacuum oven that is maintained at 35° C. Avacuum is applied on the oven and continuously drawn, maintaining thepressure at 25 inch Hg until the particles are dry. The dried particlesare analyzed by TGA (thermogravimetric analysis) and ICP. TGA indicatesthat the particles contain about 95% by weight of silver. ICP indicatesthe following composition (in % by weight): Ag 92.79, N 0.49, C 2.57, H0.27. The dried particles are formulated into an ink for ink-jetprinting.

Example 9

Silver nanoparticles prepared according to the process described inExample 1 (ranging from about 30 nm to about 50 nm in size) aresuspended in a solvent mixture composed of, in weight percent based onthe total weight of the solvent mixture, 40% of ethylene glycol, 35% ofethanol and 25% of glycerol to produce an ink for ink jet printing. Theconcentration of the silver particles in the ink is 20% by weight.

The ink had the following properties:

Viscosity* (22° C.) 14.4 cP Surface tension** (25° C.) 31 dynes/cmDensity 1.24 g/cc *measured at 100 rpm with a Brookfield DVII+viscometer (spindle no. 18) **measured with a KSV Sigma 703 digitaltensiometer with a standard Du Nouy ring method

The ink is chemically stable for 6 months, some sedimentation occurringafter 7 days at room temperature.

A Spectra SE 128 head (a commercial piezo ink-jet head) is loaded withthe above ink and the following optimized printing parameters areestablished:

Optimized Jetting Parameters (at 22-23° C.): Pulse Voltage 120 VoltsPulse Frequency 500 Hz (for up to one 1 hour of continuous operation)Pulse Rise Time 2.5 μs Pulse Width 12.0 μs Pulse Fall Time 2.5 μsMeniscus Vacuum 3.0 inches of water Performance Summary: Drop Size 39 μm(calculated volume 31 pL) Drop Velocity 0.33 m/s Spot Size (average) 70μm (on Kapton ®; measured using optical microscope)Spot Size (average) 70 μm (on Kapton®; measured using opticalmicroscope)

The deposited ink can be rendered conductive after curing in air attemperatures as low as 100° C. The ink exhibits a high metal yield,allowing single pass printing.

Using the above optimized jetting parameters, the above ink is depositedin a single pass with a Spectra SE 128 head on a Kapton® substrate andon a glass substrate to print a line. The line has a maximum width ofabout 140 μm (Kapton®) and about 160 μm (glass) and a paraboliccross-section. The thickness of the line at the edges averages about 275nm (Kapton®) and about 240 nm (glass) and the maximum height of the lineis about 390 nm (Kapton® and glass). The differences between Kapton® andglass reflect the different wetting behavior of the ink on these twotypes of substrate materials.

Single pass printing with the above ink affords a sheet resistivity offrom about 0.1 to about 0.5 Ω/sq. The printed material shows a bulkresistivity in the fully sintered state of from about 4 to about 5 μΩ·cm(about 2.5-3 times the bulk resistivity of silver).

The polymer (polyvinylpyrrolidone (PVP)) on the surface of the silvernanoparticles allows the sintering of a deposited ink at very lowtemperatures, e.g., in the range of from about 100° C. to about 150° C.The PVP does not volatilize or significantly decompose at these lowtemperatures. Without wishing to be bound by any theory, it is believedthat at these low temperatures the polymer moves out of the way,allowing the cores of the nanoparticles to come into direct contact andsinter together (necking). In comparison to its anti-agglomerationeffect in the printing ink prior to printing, the polymer in thedeposited and heat-treated ink assumes a new function, i.e., it promotesthe adhesion of the printed material to a range of polymeric substratessuch as, e.g., FR4 (fiberglass-epoxy resin) and Mylar® (polyethyleneterephthalate) and provides structural strength. As a result of thelow-temperature sintering mechanism a continuous percolation network isformed that provides continuous channels for the conduction of electronsto flow throughout the material without obstacles. This is fundamentallydifferent from the traditional polymer thick film approach, whereelectrical conductivity is established during thermal curing as a resultof polymer matrix shrinkage, inducing compressive stress on the flakeparticles and causing a reduction in their large contact resistance.

When higher-temperature sintering is performed (at about 300° C. toabout 550° C.), the polymer volatilizes. As a result, sintering willoccur and in comparison to low-temperature sintering a much denser metalmaterial is formed. This leads to a better conductivity (close to theconductivity of the bulk metal), better adhesion to substrates such asglass, and better structural integrity and/or scratch resistance.

In the low temperature sintering range (from about 100° C. to about 150°C.), the present ink can advantageously be employed for applicationssuch as, e.g., printed RF ID antennas and tags, digitally printedcircuit boards, smart packages, “disposable electronics” printed onplastics or paper stock, etc. In the medium temperature range (fromabout 150° C. to about 300° C.) the ink may, for example, be used forprinting interconnects for applications in printed logic and printedactive matrix backpanes for applications such as polymer electronics,OLED displays, AMLCD technology, etc. In the high temperature range(from about 300° C. to about 550° C.) its good performance and adhesionto glass make it useful for printed display applications such as, e.g.,plasma display panels.

Example 10 Coductivity Testing of Compositions on Various PaperSubstrates

It was found that the Ag ink composition of Example 9 yields ink jetprinted lines on Epson Gloss IJ ink jet paper that exhibit an electricresistance after annealing at 100° C. which is comparable to that of thesame ink printed on Kapton and annealed at 200° C.

In one set of tests, the following experiments were carried out:

-   -   An aqueous silver ink was jetted onto glossy IJ photo paper        (Canon), producing three groups of 4 lines; 1 set as single        pass, 1 set as double pass, and 1 set as triple pass. All three        sets were annealed on a hot plate set to 200° C. for 30 minutes.        After the anneal, the lines were tested for electrical        conductivity; all lines failed to exhibit conductivity.    -   The solvent-based Ag ink of Example 9 was printed on EPSON        S041286 Gloss photo paper to produce samples for comparison        testing with a commercially available Ag ink sample (Nippon        Paint) printed on Cannon gloss paper (model not known). Two        samples were printed, 1 coupon with a single print pass and 1        coupon with a double print pass.        -   The double pass print was annealed at 100° C. for 60            minutes.        -   The commercial Ag ink sample was cured at 100° C. for 60            minutes.        -   The single pass print was annealed at 100° C. for 110            minutes.

Both samples produced with the ink of Example 9 exhibited very goodconductivity, comparable to the same silver ink, printed on Kapton, andannealed at 200° C. for 30 minutes.

The commercially available ink yielded a conductivity much worse thanthat of the ink samples according to the present invention.

The ink of Example 9 was printed on four different substrates: (a)Kapton HN-300, (b) Hammermill 05502-0 gloss color copy paper, (c) CanonBubblejet Gloss Photo Paper GP-301 and (d) Epson Gloss Photo Paper forink-jet S041286.

The results listed in the table below confirm the superior performanceof the Example 9 ink/Epson paper combination.

Cure Approx. Temp/ Resistivity pH of Ink Substrate Time (μΩ *cm)¹Substrate³ Example 9 Kapton 200° C./  21 N/A 30 min Example 9 Kapton100° C./ 180 N/A 60 min Example 9 Epson 100° C./  16 4-5 Photo Paper 60min Example 9 Xerox 100° C./ No N/A High Gloss 60 min ConductivityExample 9 Canon 100° C./ 525 6.5-7   Photo Paper 60 min Commercial Canon100° C./ 5400²  6.5-7   Photo Paper 60 min N/A HP Premium N/A N/A 5.0Satin Gloss N/A HP Premium N/A N/A 5.0 Gloss N/A Kodak N/A N/A 5.5Premium Gloss N/A Kodak Ultima N/A N/A 5.0-5.5 N/A Canon PP101 N/A N/A6.5 N/A Canon PR101 N/A N/A 6.5-7.0 N/A Fuji N/A N/A 7.0 ¹assuming1-micron line thickness ²average based on fewer measurements than ink ofExample 9 ³pH of the substrate determined using solution indicator kit:VWR VW 5704-1 Universal Indicator solution (isopropanol 50% (v/v), NaOH,H₂O).

The results shown in the table above suggest that it is possible thatthe pH of the substrate may have an effect on the resistivity of theinks printed thereon. It is contemplated that an ink that is printed onan acidic substrate will have a lower resistivity than an ink printed ona relatively more basic substrate (compare Example 9 ink printed onEpson photo paper versus the same ink printed on Canon photo paper).

It is known that Canon photo paper is coated with basic alumina, whileEpson photo paper is coated with silica. See, e.g., Blum, A. E. andEberl, E. E., Clays and Clay Minerals 52: 589-602 (2004), the contentsof which are incorporated herein by reference in their entirety. Othercoatings, e.g., alumina doped silica, are also known in the art. See,e.g., European Patent Application EP0995718, the contents of which areincorporated herein by reference in their entirety. It is possible thatthe silica coating on the Epson paper imparts acidic properties in thepaper. Likewise, it is possible that the alumina coating on the Canonpaper imparts less acidic properties in that paper.

The reasons for the apparent effect of substrate pH on the resistivityof the ink printed thereon are not entirely clear at this time. It ispossible that, in addition to the substrate pH, other substratecharacteristics, alone, or in conjunction with the presence of certainmaterials found in the ink formulation, may be the actual factors thatcause inks printed on certain substrates to have lower resistivitiesthan when they are printed on other substrates. Such characteristics mayinclude the nature of coatings on the substrate and substrate porosity.

As mentioned above, Epson paper is coated with silica, while the Canonpaper is coated with basic alumina. Without being bound by anyparticular theory, it is possible that the PVP on the nanoparticlesadsorbs to some extent on the silica coating on the Epson paper, therebyexposing the silver nanoparticle surface. Adsorption of the PVP onsilica may be expected, since it is known that silica has a highaffinity for PVP. See, e.g., Blum, A. E. and Eberl, E. E., Clays andClay Minerals 52: 589-602 (2004), the contents of which are incorporatedherein by reference in their entirety. When the ink is subsequentlycured, the exposed silver nanoparticles may be in closer contact withone another and may sinter to form a network with decreased resistivity.

It is also possible that the substrate porosity may lead to inks withdecreased resistivity upon curing. Without being bound by any particulartheory, it is possible that the substrate porosity, in conjunction withthe presence of plasticizers (e.g., glycerol) in the ink formulation maybe aiding in the adsorption of PVP into the pores of the substrate,thereby exposing the silver of the nanoparticles. Without being bound byany particular theory, it is possible that plasticizers sufficientlysoften PVP such that the PVP can migrate onto the substrate and awayfrom the silver nanoparticles. When the ink is subsequently cured, theexposed silver nanoparticles may be in closer contact with one anotherand may sinter to form a network with decreased resistivity.

It is noted that the foregoing examples have been provided merely forthe purpose of explanation and are in no way to be construed as limitingof the present invention. While the present invention has been describedwith reference to an exemplary embodiment, it is understood that thewords that have been used are words of description and illustration,rather than words of limitation. Changes may be made, within the purviewof the appended claims, as presently stated and as amended, withoutdeparting from the scope and spirit of the present invention in itsaspects. Although the invention has been described herein with referenceto particular means, materials and embodiments, the invention is notintended to be limited to the particulars disclosed herein. Instead, theinvention extends to all functionally equivalent structures, methods anduses, such as are within the scope of the appended claims.

1-271. (canceled)
 272. A process for purifying a nanoparticledispersion, comprising the step of: filtering a nanoparticle dispersioncomprising nanoparticles, a liquid phase and impurities and/orcontaminants through a membrane having a pore size of from about 0.01 μmto about 1 μm and/or a lumen of from about 0.1 mm to about 5 mm, whereinthe membrane retains substantially all the nanoparticles in the liquidphase of the nanoparticle dispersion passes through the membrane. 273.The process of claim 272, wherein the nanoparticles comprise metalnanoparticles.
 274. The process of claim 273, wherein the metalnanoparticles comprise a metal selected from the group consisting ofgold, silver, copper, nickel, cobalt, palladium, platinum, iridium,osmium, rhodium, ruthenium, rhenium, vanadium, chromium, manganese,niobium, molybdenum, tungsten, tantalum, iron and cadmium.
 275. Theprocess of claim 272, wherein at least 70% of impurities and/orcontaminants in the liquid phase of the nanoparticle suspension passthrough the membrane.
 276. The process of claim 272, wherein at least80% of impurities and/or contaminants in the liquid phase of thenanoparticle suspension pass through the membrane.
 277. The process ofclaim 272, wherein at least 90% of impurities and/or contaminants in theliquid phase of the nanoparticle suspension pass through the membrane.278. The process of claim 272, wherein at least 95% of impurities and/orcontaminants in the liquid phase of the nanoparticle suspension passthrough the membrane.
 279. The process of claim 272, further comprisingthe step of washing the membrane retaining substantially all of thenanoparticles with a washing liquid to remove impurities and/orcontaminants from the surfaces of the nanoparticles.
 280. The process ofclaim 279, wherein the step of washing is repeated more than once. 281.The process of claim 279, wherein the washing liquid is water and/or anorganic solvent.
 282. The process of claim 279, further comprising thestep of removing the washing liquid.
 283. The process of claim 282,further comprising the step of drying the metal nanoparticles.
 284. Theprocess of claim 283, wherein the dried nanoparticles are capable ofbeing redispersed in a liquid phase.
 285. The process of claim 272,wherein the nanoparticles have an average particle size of about 10 nmto about 80 nm.
 286. The process of claim 272, wherein the membrane hasa pore size of from about 0.01 μm to about 1 μm.
 287. The process ofclaim 272, wherein the membrane comprises a polymeric material.
 288. Theprocess of claim 287, wherein the polymeric material comprises at leastone of a polysulfone, a polyethersulfone, a sulfonated polysulfone, apolyamide, and a cellulose ester.
 289. The process of claim 272, whereinthe membrane comprises a ceramic material.
 290. The process of claim289, wherein the ceramic material comprises an oxide of at least one oftitanium, zirconium, silicon and aluminum.
 291. The process of claim272, wherein the membrane has a molecular weight cutoff in the range offrom about 10,000 to about 1,000,000.
 292. The process of claim 272,further comprising the step of treating the surfaces of thenanoparticles of the nanoparticle dispersion with an absorptivesubstance prior to the step of filtering.
 293. The process of claim 272,further comprising the step of increasing the concentration of thenanoparticles in the nanoparticle dispersion prior to the filteringstep.
 294. The process of claim 293, wherein the step of increasing theconcentration of the nanoparticles is performed by drawing thenanoparticle dispersion though a membrane.
 295. The process of claim272, wherein the nanoparticles are produced by rapid mixing of asolution of at least about 0.1 mole of a metal compound that is capableof being reduced to a metal by a polyol with a heated solution thatcomprises a polyol and a substance that is capable of being adsorbed onthe nanoparticles.
 296. The process of claim 295, further comprising thestep of precipitating the nanoparticles in the nanoparticle dispersionprior to the filtering step.
 297. The process of claim 272, wherein thenanoparticle dispersion further comprises one or more adhesion promotersand/or humectants.
 298. The process of claim 272, further comprising thestep of creating a printing formulation, ink or paste from the filterednanoparticle dispersion.